Rhendak the crazed
For Glade and Glory
I’m pretty sure Rhendak isn’t a world boss. He’s a boss for the mini-dungeon Flame Temple Tombs, and he has a loot chest that you can open after he’s dead.
The last time I killed him (which admittedly was months ago), he DID give a bonus chest like the other World Bosses, in addition to the big ground chest that spawns after his death. However, since few people do his mini-dungeon, this fact is largely unknown by players.
I honestly don’t know if he’ll still stay as a World Boss after the 15th patch, but I think it’s EXTREMELY likely that beating him is a requirement for unlocking a trait under the new system.