If you're not familiar with DnD5e, then spellcasting in BG3 is probably unlike any game you've ever played. It may seem overwhelming, but there is a method to this madness. This guide starts with the most basic features of BG3 spellcasting, and adds new information in a logical step-by-step process. This guide is not a dissection of 'the best' caster classes or builds, nor is it a detailed breakdown of all 'the best' spells. DnD5e and BG3 pros are unlikely to find much here they don't already know. This is a guide for total noobs! Let's begin!

This section offers a brief summary of caster classes with some of my own opinions regarding their features.

Wizards
The most flexible caster in BG3, Wizards can do it all. That's not to say they're the 'best' caster class, but certainly they're the most versatile because of their expansive class spell lists. Wizards can also easily learn new spells by copying them from Spell Scrolls that seem to grow on trees in BG3. Arcane Recovery means Wizards have a bit more gas in their tank so they can continue casting a few more spells when other casters are running on empty. The various Schools of Magic subclasses all have interesting perks that add a unique flair for Wizards. IMHO, the Abjuration School is one of the best options because of Arcane Ward, which gets even stronger in the end game.


Sorcerers
Less versatile than Wizards generally, and with fewer subclass options, Sorcerers are still awesome casters. They get Sorcery Points which are similar to a Wizard's Arcane Recovery, and they also get Metamagic which lets them customize their spells in powerful ways. You want to know what's cooler than Banishing an enemy into another plane of existence - Banishing two enemies into another plane of existence. But perhaps best of all - Sorcerers don't need to prepare their spells. Meaning, every spell a Sorcerer knows is always ready for casting, waiting at their fingertips to be unleashed. Since Sorcerers use CHA, they're also much better at social interactions than Wizard-nerds, tight-arsed Clerics, and patchouli-stinking Druids.


Druids
This class is as equally complex and unique as the Warlock, but not necessarily because of their spellcasting. The Wild Shape ability probably spawns more questions and caveats than any other class ability in the entire game. However, there's no denying its power and versatility. While this feature is the first thing that comes to mind when most people think of Druids, they can also be incredibly strong casters who use Wild Shape primarily for utility and sneaking rather than always mauling enemies with their bear claws. While Druids don't have as an extensive catalog of spells to learn as Wizards, they can cast a powerful array of damage, healing, control, buff/debuff, and utility spells that rivals any Wizard. They also have more armor and weapon proficiencies than Wizards and Sorcerers if they want to mix it up with a little bit of toe-to-toe action.


Clerics
This class generally lacks the volume of damage spells that Wizards and Sorcerers enjoy, but the damage spells they do have are some of the best in the game. Clerics are generally known for their healing and buffs/debuffs, but don't ever mistake them as pure healers required to put a band-aid on all your boo-boos. With their Domains granting them special powers and spells, Clerics are often more beneficial allies as combatants than stretcher bearers. IMHO, the Light Domain is one of the best because it provides several damage spells Clerics can't normally learn and grants Warding Flare as a fantastic defensive option. Their ability to Channel Divinity gives Clerics powers that other casters can only dream of. All around my favorite caster class by far.

Warlocks
Some players might be put off by the Warlock's lack of Spell Slots even at high class levels, but IMHO this class has strengths that make it a fun choice. The possible combinations of subclasses, Pacts, and invocations allows for a great variety of options. Perhaps not as noob friendly as some other caster classes, Warlocks are often part of multiclass builds because they offer so much at low levels. Admittedly, I struggled with my first couple attempts to create and play my custom Warlocks, but the more I play this class the more I enjoy it and the more effective I make my character. It takes some getting used to.


Bards
I have to admit, I really hate the concept of the stereotypical, outlandish 'performer' Bard because IMHO the class is so much more than that. Another solid multiclass choice, Bards can hold their own in martial combat or behave more like a pure Wizard or Sorcerer caster. They have a good array of skills, and are fantastic as buff/debuff support characters with spells and Bardic Inspiration. I have a lot of fun with the combat options from College of Swords, but the other Colleges are also really strong. In fact, it's almost impossible to make a bad Bard build because everything they offer is a good choice.


Rangers
Rangers don't have any class or subclass features which allow them to really lean into spellcasting. It's just not their thing. It's not that surprising since Rangers are half-casters. But, with some carefully selected spells, a Ranger's spellcasting can help turn the tide of battle. A few notable spells they have include Lesser Restoration, Silence, Protection from Energy, and of course Hunter's Mark, but they also have solid buff spells like Longstrider and Pass Without Trace. I suggest choosing Natural Explorer: Beast Tamer for the Find Familiar spell. It's a great early game summons spell, especially if you don't plan on going with the Beast Master subclass.


Paladins
This class has A LOT of concentration spells, so War Caster is a feat worth serious consideration for this melee character. However, most Paladins conserve their Spell Slots as fuel for their Divine Smites. Oath spells significantly expand their spellcasting options and should be a major factor when selecting their Oath. A few notable spells include Compelled Duel (one of the few 'aggro' options in BG3), Sanctuary, Aid, Heroism, Lesser Restoration, and of course all the various Smites. Compared to Rangers, this class has significantly more spellcasting options should they choose to use them.

We'll begin by identifying some of the most basic issues of spellcasting.

Spell Levels
Spells have levels distinct from class/character levels. Meaning, just because your Cleric is 2nd level doesn't mean you can cast 2nd level spells yet. Gaining access to higher level spells requires gaining multiple levels in your spellcasting class. There are '0 level' Cantrips (we'll get to Cantrips a bit later in this guide), and up to 9th level spells. However, because of the BG3 cap on class levels at 12, spells in this game are capped at level 6. If you want to cast 7th, 8th, and 9th level spells you'll need to join an actual DnD 5e campaign!


Spellcasting Ability Scores
Each caster class uses a certain Ability score to determine their spell repertoire and how difficult it is to resist their spells.

Wizards - INT
Clerics, Druids, and Rangers - WIS
Sorcerers, Bards, Paladins, and Warlocks - CHA

It's important to note that spells are not intrinsically linked to a specific Ability Score by themselves. Meaning, there are not categories of INT spells, WIS spells, and CHA spells. Rather, the primary spellcasting Ability Score for a spell is determined either by the Race or Class of the caster. For example, a High-Elf that selects Firebolt as a cantrip for their racial feature will always use INT when casting that version of Firebolt, even if they're a Sorcerer that also selected Firebolt for their class cantrip and uses CHA for that selected version of Firebolt. Yes, a character can have different versions of the same spell that use different ability scores depending on their race and class combinations.

As another example, Charm Person is not a CHA spell by itself. It's available to many caster classes, so when cast by a Cleric Charm Person is impacted by WIS, when cast by a Bard it's impacted by CHA, and when cast by a Wizard it's impacted by INT. In other words, there are simply class spells, and while numerous spells crossover multiple classes, there are some spells which are exclusive to certain classes.


Resting
Casting spells is mentally taxing. You can't spam Fireballs every turn of combat. Casters have a fixed number of spells they can cast before they need to rest, known as Spell Slots (we'll get to that soon). After resting, they regain their Spell Slots and can cast spells again. It's important for casters to learn when it's the best time to cast their spells because taking a rest after every combat encounter is an incredibly inefficient and expensive way to play the game. Every caster regains spells after a long rest, and Warlocks regain spells after a short rest, too.

Full-Casters
Full-casters learn new higher level spells for every two class levels. For example, a Wizard learns 2nd level spells as a 3rd level Wizard, and then learns 3rd level spells as a 5th level Wizard, etc. Because of this rapid spell level progression, full-casters can cast more spells than other caster classes. Wizards, Sorcerers, Clerics, Bards, and Druids are full-casters and can eventually learn 6th level spells as an 11th level full-caster class.


Half-Casters
Half-Casters learn spells at a much slower rate and have a spell level cap of 3rd level. This is because half-casters also have significant martial skills and don't rely as heavily on their spells as full-casters. In fact, half-casters don't learn any 1st level spells until they're 2nd level in their class, and have to be 5th level in their class in order to learn 2nd level spells. Additionally, half-casters do not know any Cantrip spells by default. Half-casters can cast 3rd level spells from their 9th to 12th class level. Rangers and Paladins are half-casters.


Arcane Trickster & Eldritch Knight
Arcane Trickster and Eldritch Knight have even less spellcasting prowess than Half-Casters. However, even a handful of carefully selected low level spells add an immense amount of utility and options in combat throughout the entire campaign. If you don't want to give up the awesome high level Rogue and Fighter class abilities, but also want to cast spells, definitely try the Arcane Trickster or Eldritch Knight instead of multiclassing!


Warlocks
And then we have Warlocks. Ok, here we go. Warlocks have the most unique spellcasting mechanics in BG3. By default, they can learn Cantrips, 1st to 5th level spells, and one special 6th level spell, but the number of spells they know is less than full-casters, and the amount of spells they can cast before needing to rest (Spell Slots) is even less than half-casters. Well, that sounds terrible! But wait, it gets much, much better.

The trick is, as mentioned previously in this guide, Warlocks regain all of their spells after taking only a short rest, while every other spellcasting class requires a long rest. Also, Warlocks always cast their spells at the highest spell slot possible to make their spells more powerful, a mechanic known as 'upcasting'. We'll get to spell slots and upcasting in a bit.

Spells Known
Every caster class has a certain number of spells they know. As they level up they can learn new spells. Some caster classes, like Clerics, Druids, and Paladins automatically know every available spell for their class when they level up. Other caster classes, like Wizards, Sorcerers, Bards, Warlocks, and Rangers select a handful of known spells at character creation, and two class spells to know from the available options when they level up. This is an incredibly important distinction I think is often overlooked when players evaluate the strengths and weaknesses of caster classes.

Additionally, Clerics know extra spells from their Domain, and Paladins know extra spells from their Oath. Domain and Oath spells don't count against the normal limit of spells a caster can prepare (we'll get to prepared spells next). That means Domain and Oath spells are always ready for casting in addition to the spells a Cleric or Paladin chooses to prepare after a long rest or at level up. Do not confuse this with gaining extra Spell Slots. Domain and Oath spells still require a Spell Slot just like other spells.

Wizards can increase their spells known by paying gold to copy new Wizard class spells from Spell Scrolls and are the only class capable of learning spells in this manner. Sorry, Wizards can't learn Cure Wounds from spell scrolls, or any other spell not included on the default Wizard class spell lists.

For Sorcerers, Bards, Warlocks, and Rangers it's incredibly important to carefully select known spells when they level up because that's the only way they can learn new spells by default (not including subclasses, Feats, Invocations, etc).

Selecting Low and High Level Spells Known
When casters reach a class level milestone that allows them to learn higher level spells, I suggest they also consider spells from lower levels as an option. As I'm sure you've noticed, the number of available spells at high levels shrinks a lot compared to low spell levels. Just because you can pick new higher level spells when leveling up doesn't mean you have to. There are probably still a few really good low level spells that perhaps were not a top priority in the early game, but might be much more useful at higher levels.

Also consider the number of new high level Spell Slots you get. There might be better options than picking more high level spells than you have Spell Slots to cast them. For example, if you only have one new high level Spell Slot, but you know two high level spells, you're always juggling which is the best spell to prepare or cast until you level up again and get more of those Spell Slots. Yes, there are workarounds for this conundrum, like Arcane Recovery, but sometimes it's like putting the cart before the horse. Just because a spell is higher level doesn't automatically make it better than lower level spells.


Spells Prepared
After taking a rest and at level up, Wizards, Druids, Clerics, and Paladins select a certain number of spells from their pool of known spells, and prepare them for actual casting. In DnD5e this is roleplayed in the form of studying a spellbook, praying to a god, or meditating about nature for a short period of time. Wizards, Druids, and Clerics can prepare a number of spells equal to their caster class level + their primary spellcasting ability modifier, while Paladins prepare spells equal to half their caster class level + their primary spellcasting modifier. In BG3 this is shown in the Spellbook tab of your character info menu. It's basically like preparing a menu of spells you might want to cast that day.

Bards, Warlocks, Rangers, and Sorcerers do not go through this process to prepare spells. Essentially, all of their known spells are always prepared. The trade-off is that they generally know fewer spells.


Spells Known and Spells Prepared Caster List

Wizard - limited number of spells known, limited number of spells prepared

Sorcerer - limited number of spells known, all known spells always prepared

Bard - limited number of spells known, all known spells always prepared

Druid - knows all class spells, limited number of spells prepared

Cleric - knows all class spells, limited number of spells prepared

Warlock - limited number of spells known, all known spells always prepared

Ranger - limited number of spells known, all known spells always prepared

Paladin - knows all class spells, limited number of spells prepared


Spell Slots
THIS is how many spells a caster can actually cast before needing to take a rest. Depending on their class level, each caster class has a certain number of 1st Level Spell Slots, 2nd Level Spell Slots, 3rd Level Spell Slots, and so on. Full casters can get up to four spell slots for 1st lvl spells, but only one spell slot for 6th lvl spells. So, classes that don't automatically know all of their class spells when leveling up, like Wizards, Sorcerers, Bards, Warlocks, and Rangers must carefully select spells at higher levels.

Let's compare spells and spell slots for a high level Wizard and Cleric. A 12th level Wizard can know as many as four 6th level spells by default (they can potentially know all 12 spells via copying from spell scrolls), but with only a single 6th level spell slot, they can still only prepare one of those spells at a time. A 12th level Cleric automatically knows all six of their 6th level class spells, but they too only have a single 6th level spell slot.

Spell Slots in BG3 are represented as small check boxes on the Action Bar interface. When you cast a spell, one of your Spell Slot check boxes of the appropriate Spell Level loses its color and becomes white. This is how you visually keep track of your amount of remaining Spell Slots. You can see all your available spells to cast by clicking on the Action or Bonus Action boxes, or you can click on your 1st Level Spell Slot check boxes to see only your 1st Level spells, click the 2nd Level Spell Slot check boxes to see your 2nd level and lower spell, etc. In the Upcasting section we'll learn how these check boxes make that mechanic easy to use in game.

Most spells that target one or more creatures directly or within an AoE, either make an Attack roll to hit the target, or force the target to make a Saving Throw to possibly resist the spell's effect. To know whether a spell requires an Attack roll or Saving Throw, look at the detailed description of it. Near the bottom of that infographic is either 'Attack' or something like Saving Throw DEX, or Saving Throw CON, etc. AFAIK, spells never require both an Attack roll and a Saving Throw.

Spell Attacks
A spell Attack roll is just like making a melee or ranged weapon Attack roll. However, instead of using STR or DEX to make the Attack roll, the caster uses their primary spellcasting Ability (INT, WIS, or CHA) for the Attack roll. Attack spells have a specific range, from melee up to 18M or 60ft. They are subject to the same modifiers as weapon attacks like Advantage/Disadvantage, high ground, Attack buffs/debuffs, critical failure/success, etc. So, when a caster encounters an enemy with high Saving Throws, maybe they should try their Attack spells instead.


Spell Saving Throws
Some spells automatically hit their target, or create an AoE, and instead force it to make a Saving Throw. A Saving Throw is a d20 roll modified by the target's appropriate Ability score and perhaps Saving Throw proficiency for that Ability. The appropriate Saving Throw Ability is listed in the spell infographic. Casters do not make the Saving Throw roll, only the target makes that roll. If the target does not roll equal to or higher than the spellcaster's DC (we'll get to that in a bit), then bad things happen to them as described in the spell's infographic.

Many damage spells, like Fireball, still deal half damage even if the target succeeds on the Saving Throw! Some spells, known as Save or Suck spells, do absolutely nothing if the target succeeds on the Saving Throw. However, many Save or Suck spells are way OP if the target fails the Saving Throw.

Before casting a spell with a Saving Throw, it's strongly recommended that the player 'Examine' their target and look at the appropriate Ability Score/Saving Throw for the spell they intend to cast. For example, if your target has a high WIS Ability Score/Saving Throw, then perhaps your Bard should reconsider casting Crown of Madness on them, and instead either use an Attack spell or a Saving Throw spell that exploits the target's lowest Ability Score.

In BG3, the % you see when you select a target with a spell that requires a Saving Throw is the chance your spell will succeed. So, just like with Attack rolls, you want to see a high % chance for your targets of spells with Saving Throws.


DC (Difficulty Class)
If a spell requires a target to make a Saving Throw, the result of that roll must be equal to or higher than the caster's DC (Difficulty Class). This represents how easy or hard it is to resist the spell's effect. The formula for BG3 DC is 8 + Proficiency Bonus + Primary Spellcasting Ability Score Bonus.

For example, your 3rd level Bard with a 16 CHA attempts to cast Blindness on a Bugbear. The spell DC is 8 + 2 + 3 = CON DC 13. This hypothetical Bugbear has a 16 CON, so it rolls a d20 and adds +3 for its CON ability score bonus for a final of d20+3. By simply rolling a 10 or higher on the d20 for a total of 13+, the Bugbear succeeds on the CON Saving Throw and completely resists the effects of the Blindness spell.

So, unless a target has a naturally low Saving Throw for a particular Ability Score, or it's suffering some sort of Saving Throw debuff, you can see that rolling a successful Saving Throw is not too difficult. Often, the odds are about 50/50. If you plan on using lots of Saving Throw spells for your caster, you should mostly invest in Ability Score improvement Feats for your primary spellcasting Ability, and/or find ways to debuff enemy Saving Throws.

Some spells resolve immediately while other spells last for a certain duration. Some duration spells have an extra requirement to maintain them called Concentration. Spells with this requirement will have it listed near the bottom of the spell infographic.

A caster can only maintain one Concentration spell at a time. For example, if a Cleric is Concentrating on Bless, and then casts Shield of Faith, the Bless spell and its effects cease immediately. There's no way for one caster to stack multiple Concentration spells or carry over the effects of a previous Concentration spell while casting a new Concentration spell.

It's generally a bad idea for a caster to load up on lots of Concentration spells, after all, they can only maintain one at a time. Most casters typically have one or two Concentration spells ready for combat, and maybe another non-combat Concentration spell.

Concentration Checks
If a caster takes any damage from any source while maintaining Concentration, they must make a Concentration Check to see if they continue to maintain the spell or lose it. A Concentration Check is actually a Constitution Saving Throw. The DC of the Saving Throw is either DC 10, or half the amount of damage if the result is more than 10. So, 20 points of damage or less is always DC 10, while 22 points of damage is DC 11, and 30 points of damage is DC 15, etc. If the caster fails the Concentration Check then the spell ceases immediately and all its effects stop. For this reason, CON is actually a vital Ability Score for any caster who intends on casting Concentration spells.


Incapacitated
If a caster with Concentration is incapacitated[bg3.wiki], they automatically lose Concentration. Spells like Sleep and Tasha's Hideous Laughter incapacitate the target. Paralysis[bg3.wiki] and other status effects may also incapacitate the target.


Maintaining Concentration
Maintaining Concentration is particularly difficult for casters like some Clerics and Paladins who frequently engage in melee combat and are likely to take damage. However, there are a few things casters can do to improve the odds on their Concentration Checks...

The War Caster[bg3.wiki] feat is a popular choice for many casters because it imparts Advantage on Concentration Checks.

The Resilient: CON[bg3.wiki] feat is another solid option for this issue if your character lacks proficiency in CON Saving Throws.

Buffs that improve Saving Throws or CON Ability checks are helpful.

Of course, not taking any damage by staying out of the combat mosh pit is always a good idea, too.

If a Concentration spell is truly critical for the success of a combat encounter, I'll cast Sanctuary [bg3.wiki]on the caster to help prevent direct damage on them.

This powerful mechanic allows casters to utilize low level spells throughout the entire campaign. Every caster class is capable of 'upcasting' spells if they have 2nd level or higher Spell Slots. A caster's highest level Spell Slots are few and precious, but eventually most casters have plenty of extra low and mid-level Spell Slots. Upcasting allows casters a lot of flexibility with all those Spell Slots. Cantrips cannot be upcasted, but several of them are innately more powerful when cast at higher caster class levels.

A caster's 2nd level and higher Spell Slots are not limited to only casting 2nd level and higher spells. Every spell can be cast using a higher level Spell Slot, but certain spells can be cast at a higher level Spell Slot for a more powerful effect than its normal spell level. Otherwise, low level spells like Cure Wounds and Magic Missile would suck really bad. If a high level caster wanted to, they could use all of their Spell Slots (not just their 1st Level Spell Slots) to cast Charm Person over and over again. Upcasting Charm Person doesn't make it more powerful, but a caster isn't limited to only using 1st Level Spell Slots to cast it.

However, do you know what's better than casting a normal Fireball, how about upcasting it to a 6th level Fireball for a boatload more damage! The vast majority of spells that roll dice as part of the effect can be upcasted to roll more dice. Some spells with a certain number of targets can be upcasted to effect more targets. Some spells that only effect a certain number of enemy hit points can be upcasted to effect more enemy hit points. Some spells that grant temporary or increased Max Hit Points can be upcasted for more Hit Points. You get the gist.

To upcast a spell in BG3, click on a 2nd Level Spell Slot or higher check box on the Action Bar, and select a lower level spell. Its effects, if possible, will be automatically increased (number of dice, targets, hit points, etc.) when cast this way. Warlocks don't need to do this because their upcasting happens automatically. We'll get to that next.

Warlock Upcasting
By default, every Warlock spell other than Cantrips, is automatically upcasted to the highest Spell Slot Level available, they don't get to choose whether or not to do it. The Warlock Spell Slot chart[bg3.wiki] shows the Spell Slot Level for each Warlock class level.

For example, a 7th level Warlock upcasts all their spells to a 4th level Spell Slot. That means after taking a couple of short rests to regain their spells, a Warlock can upcast 6 spells to 4th level before finally needing a long rest. By comparison, a 7th level Wizard only has a single 4th level Spell Slot, so if they upcast just one spell to 4th level, that 4th level Spell Slot is gone until they take a long rest or use a lot of their Arcane Recovery. This is to balance the fact that a Warlock's number of available spells is severely limited compared to a Wizard's. It's a trade-off between versatility and power.

Cantrips are the lowest level spells in BG3. Technically they don't even have a level, so I refer to them as 0 Level spells. Many new players underestimate the value and power of cantrips, perhaps because they're salivating over the chance to nuke a horde of Gnolls with a Fireball. However, cantrips are some of the most valuable spells in a caster's repertoire.

Unlike higher level spells, cantrips do not use a spell slot when cast. This means a caster can spam these spells to their heart's content. Even high level casters often use cantrips for combat unless there's really a need for more serious measures. Think of it this way, infinite cantrips help preserve your finite Spell Slots.

Another nice perk for cantrips is that many of them deal more damage at higher caster levels without needing to upcast them! Most damage cantrips gain another damage dice every five caster levels. This is an incredibly efficient damage source for casters who may want to preserve their Spell Slots for control or buff/debuff spells instead.

Most casters, if possible, select one damage cantrip with an Attack roll like Firebolt, and another damage cantrip with a saving throw like Acid Splash. Utility cantrips like Light and Mage Hand are great, but don't scoff at Bone Chill or Sacred Flame.

There are a small number of spells in BG3, like Find Familiar[bg3.wiki], with an important tag called Ritual Spell[bg3.wiki]. What makes Ritual Spells distinct from other Spells is that they can be cast outside of combat without using a Spell Slot. This makes them incredibly efficient spell options, especially for Half-Casters and Warlocks. Ritual spells can also be cast during combat, but will use a Spell Slot.

Generally, Ritual Spells seem a bit underpowered compared to non-Ritual spells and may cause some players to bypass them for selecting known spells at level up or at character creation. But, considering they are basically 'free' spells when cast outside of combat, and many Ritual Spells have a long duration, it's never going to hurt a caster to have two or three Ritual Spells in their repertoire.

In another major departure from DnD5e rules, in BG3 a caster can cast an Action spell and Bonus Action spell of any available Spell Level in the same turn without restrictions. This means a well prepared caster should have spells for both Actions and Bonus Actions, and even Reactions, too. This allows a caster to truly flex their spellcasting prowess in one turn for some great combos.

The vast majority of spells are Actions. So, Bonus Action spells deserve extra scrutiny for a caster's repertoire. It's worth noting that Sorcerers can use Metamagic to convert any Action spell into a Bonus Action spell. This allows them to cast the same spell twice in one turn or make a two spell combo that casters can't normally cast. This feature by itself is good enough reason to multiclass into Sorcerer (yes, we all know about that other reason to multiclass into Sorcerer).

For example, a very simple Action + Bonus Action spell combo for a low level Cleric is to cast Bless on their allies as the Action spell, followed by Sanctuary as the Bonus Action spell. This not only prevents the Cleric from being directly attacked, it's also great protection for the Concentration of the Bless buff.

As another example, a Wizard may find itself threatened by a relentless, but rather dimwitted, melee attacker. So they cast Misty Step as a Bonus Action to escape, and then casts Fear on the weak minded brute as an Action to debuff it and prevent it from closing on them again.

There aren't a lot of spell feats, but most of them are pretty good choices for a variety of reasons. We'll go through them in alphabetical order for some of my own opinions regarding their usefulness.

Elemental Adept[bg3.wiki]
This spell ensures that your 'go to' damage spell deals as much damage as possible. Not usable all of the time, but considering how many monsters and NPCs have various types of damage resistance, it's a megastar when you need it most.

Magic Initiate[bg3.wiki]
This feat allows cross-pollination for casters (or any class actually) that want to expand their spell repertoire without multiclassing. This feat provides its best value when choosing class exclusive spells, or spells not normally available for your current class. * cough * Eldritch Blast * cough * Sure, a Cleric could use this to get more Cleric Cantrips and another 1st level spell, but that doesn't seem like a great deal compared to many other Feats.

Ritual Caster[bg3.wiki]
Since this requires 13 INT or WIS, a lot of non-caster classes and CHA casters won't qualify for it. Like Magic Initiate, it's a good way to expand a caster's spell repertoire without multiclassing, but it's also an option for non-casters to snag a couple of useful long duration buffs, too. A Monk able to self-buff with Longstrider and Enhance Leap combos nicely with their Step of the Wind: Dash for some Crouching Tiger, Hidden Dragon style maneuverability. If you want to see what it's like to play a high level Monk in DnD, you should really watch that movie.

Spell Sniper[bg3.wiki]
Provides a cantrip from a prepared list and lowers the Critical Hit roll for spells to 19 instead of 20. Keep in mind the critical hit applies to all your spells, not just the cantrip you select with the feat. Considering how brutal the RNG is in this game, Spell Sniper is not a top pick for my casters.

War Caster[bg3.wiki]
Another top pick for a lot of casters. Advantage on Concentration Checks is HUGE! A Reaction: Shocking Grasp as an Opportunity Attack without actually needing to learn the spell is a nice 'lil something extra. However, this feat doesn't grant Shocking Grasp as an available known spell. Meaning, unless a caster has already selected Shocking Grasp as a known spell at character creation or at level up, they can only cast it as a Reaction trigger from this feat.

There are three main categories of spells for combat: Damage spells, Battlefield Control Spells, and Buff/Debuff spells. A good combat caster should have options for all three categories. Another consideration is concentration spells. As mentioned in the previous section, concentration spells have their own host of issues. It's generally a bad idea for casters to have lots of combat spells that require concentration. Usually, just one or two concentration spells for combat is enough.

Damage Spells
Spells in this category are pretty obvious. They deal damage to one or more enemies. Spells like Burning Hands[bg3.wiki], Dissonant Whispers[bg3.wiki], and Melf's Acid Arrow[bg3.wiki] that still deal half damage even on a successful Saving Throw or a missed Attack roll should be high priority.

* cough * Magic Missile [bg3.wiki]* cough *

Of course there's everybody's favorite spell, Fireball[bg3.wiki]. Just be careful not to blow up your friends. Spells like Vicious Mockery[bg3.wiki] that deal a little damage and impart a significant debuff are really about the debuff rather than the damage.

A well prepared caster should have a variety of damage types for their prepared damage spells. Many monsters have resistance or even immunity to certain types of damage. If you only have Fire damage spells to cast, and an enemy is immune to it, then you're going to have a very bad time. Finally, always ask yourself this question when evaluating damage spells, 'What happens if this spell doesn't hit, or the target makes the Saving Throw?'.

Battlefield Control Spells
These spells may sometimes deal a bit of damage, but their main purpose is to control enemies or alter the environment to make the situation more difficult for enemies. In many encounters, 'setting the stage' with such spells before your Wizard starts slinging Fireballs is the best option. Control spells may also provide a significant benefit for martial characters in your party, like imparting Advantage vs the target, and act kind of like a buff in that regard.

Spells that Alter the Environment
Spells like Web[bg3.wiki], Grease[bg3.wiki], and Entangle [bg3.wiki]make movement incredibly difficult, if not impossible. Imagine a scenario where your strong, Athletic characters stand right at the edge of the Web, Grease, or Entangle AoE and simply Shove enemies back into it as your ranged attackers and spell casters pick them apart. Sounds like fun!

Spells that Control Enemies
Spells like Command[bg3.wiki], Sleep[bg3.wiki], and Hold Person[bg3.wiki] can take an enemy completely out of the fight for a short time. Higher level spells like Banishment[bg3.wiki] and Hold Monster[bg3.wiki] completely turn the tide of battle. These spells do nothing when they fail, but are way overpowered when they succeed. A well prepared caster should always have at least one or two solid control spell available.

Buffs/Debuffs
The simplest examples of Buffs and Debuffs are the spells Bless [bg3.wiki]and Bane[bg3.wiki], or the ever popular Haste [bg3.wiki]and Slow[bg3.wiki]. Most buff and debuff spells require Concentration and those casters need to be careful to maintain it. A prolonged buff or debuff can often determine the outcome of a battle, especially when they impact multiple targets. Many buff spells have a duration of 'until Long Rest' meaning you should cast them outside of combat so you don't waste Actions casting them in combat. Even buff or debuff spells with shorter durations of 10 turns or less should be cast the turn before you begin combat, if possible. Switch to Turn Based Mode in order to do this.

One of the trickiest things for new players to learn is choosing which spell to cast during combat and which target to cast it on. I generally, but not always, target casters first, then ranged enemies, then melee enemies.

These are the criteria I use to evaluate each turn of an encounter before deciding which spell to cast and which target to cast it on...

Is our group outnumbered or do we outnumber the enemies?
When my group is outnumbered, I'm either going to cast a damage spell on a target(s) I'm confident I can kill or seriously injure, or I'm going to cast a control/summon spell to hinder as many enemies as possible, or the single strongest enemy. Whatever is the best way to help even the odds. I'll continue casting spells like this for as long as our group is outnumbered. However, if we outnumber the enemies, I'm not going to waste my higher level spells when the martial characters in our group can handle things (assuming they're not about to die). In that case I'm more likely to go with a damage Cantrip.

How powerful are our foes?
This has to do with Spell Slots and upcasting spells. The power of my spells should usually match the power of the enemies. Versus low power enemies, I'm much more likely to cast Cantrips and let the martial characters do the heavy lifting. Versus powerful foes, or a singular powerful foe, I'm much more likely to use higher level Spell Slots. Sure, casting Fireball is lots of fun, but wasting powerful magic on low threat enemies is a really inefficient and costly way to play BG3.

How far away is the enemy?
This also has to do with positioning of my casters. Unless they are melee range casters, I always try to keep my casters far enough away from enemies that they have to Dash to reach melee range of my casters. If the enemy is far away from our group, meaning the enemy needs more than a normal movement to engage with our group, then I'm more likely to cast an environmental control spell (Web, Grease, Entangle, etc) between our groups or directly targeting an AoE where the enemies are standing. Inhibiting the enemy's ability to get close to your group is usually a winning combat tactic.

Are my companions injured or at/near full health?
I only use healing spells on allies that are dying or near death. If they're at full health I'm more likely to buff them with Temporary Hit Points or Increased Max Hit Points, like Aid. Maybe the best option is to cast Sanctuary on a severely wounded character so they can take a brief 'time out' and drink a couple of healing potions without getting ganked.

Do these enemies have high Saving Throws, high AC, or both?
I always 'Examine' my foes before targeting them with spells so I can assess their Saving Throws, AC, Resists, and Immunities.

Versus targets with high Saving Throws, I'm more likely to target them with Attack Spells.

Versus targets with high AC, I'm more likely to target them with Saving Throw spells against their weakest Ability Score.

Versus targets with high AC and Saving Throws, I'm more likely to buff my allies, cast a Battlefield Control spell like Grease to inhibit enemy movement, or use an 'always succeeds' spell like Magic Missile or Sleep (if their HP is low enough).

As you can see, spellcasting in BG3 is complex. There are eight spellcasting classes that cast spells and learn spells in a variety of ways. There is a huge variety of spells capable of an incredible range of damage, effects, and targets. Mastering everything spell related in BG3 is something that will take a lot of time and experience. I've been playing DnD for nearly 20 years, and there are still spells I've never used and caster subclasses I've never tried. If you're struggling with spellcasting, you may need to immerse yourself a bit more in the intricacies of certain classes and mechanics to ease your frustrations. This stuff ain't easy.