Artifact crystal is needed to acces some bosses that can be found in different spawns, you will need a artifact crystal to pull the lever in the room and spawn the boss that can drop some cosmetic items.
You can find the bosses in these spawns:
Hellspawn lvl 200+
Serpent Spawn lvl 200+
Nerubian lvl 1200+
Plaguesmith lvl 1200+
Deepling Warrior lvl 1200+
The Hive lvl 600+
Lizard Chosen lvl 600+
Cursed Ape lvl 600+
Braindeath lvl 600+
More will be added for lvl 3k+ spawns soon.
Fonte base:
Mytheria Wiki