Uber Diablo Quest
When Uber Diablo spawns in a game the message "Evil Walks the Earth" appears. The task for players then is to find him; he can appear in the place of any SuperUnique in the game. He is substantially more powerful than the Diablo you face at the end of Act Four, with far more hit points, higher resistances, and the ability to deal out a lot more damage. He can also use a special version of the Druid's Armageddon spell, which causes fireballs down all around him.
Uber Diablo will only spawn on Hell difficulty.