Obsessive Devotion

After Golem, four Cultists gather at the dungeon's entrance - kill the lot and the Ancient Cultist boss appears. The fight is a dance of fireballs, ice shards and lightning; the one rule is don't hit the decoys during the ritual phase or you'll summon a Phantasm Dragon to make life worse. Hit the real one (the one that took damage) and repeat. Death starts the Lunar Events immediately (Star Destroyer), so arrive prepared.