Uber Diablo Quest
Uber Diablo is a special event boss that was added to the game in v1.10. He can only spawn on the realms, and appears after a given number of Stones of Jordan have been sold on that particular server.
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, so all values are for that difficulty.