For this boss you need to farm four Star Shards.
They are random spawn in Shrouded Wood Expedition or in Frostmarch.
They look like this (inside an egg):

Boss portal location (pink marker):

After that, place shards in the four totems and open a portal at the top of the stair:

Video:

Boss spawns in a random event "Seeds of Corruption" near Frozen Bog portal in Frostmarch. Here you need to clear 3 totems, then jump into portal and kill the boss.
Video:

Simply come to this spot on the map (near two chests) and enter the train, closest teleport - "Abandoned Outpost"
Train:

And here he is:

No video for this one :(

To summon this boss, need four Primal Fragments.
You can get them from completing event in Cindered Village. Closest teleport - "Frozen Bog"

Find 4 fire runes on houses, then kill the elite
Runes:

Elite:


After this, teleport to Highlands - Ironstone Keep and follow the video below