U'zhul is a daemon sealed within the Slayer of Kings, granting Archaon his immense power. He burned Starholm to ashes after he was unwittingly released by its priests trying to banish Archaon. Archaon managed to recaptured the daemon after the location was destroyed.[1]
Sources
- 1: Battletome: Everchosen (2015), pg. 29-32