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 Stone of Jordan unique rings have been sold on that particular server. With this in mind, remember that this is not a part of the Pandemonium Event, you'll fight Pandemonium Diablo in that battle!
When Uber Diablo spawns in a game the message "Evil Walks the Earth" appears. Uber Diablo should appear in place of the next SuperUnique to spawn, though this can be tricky to determine in a large game with multiple players in multiple locations. 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.
You can find more about his prowess in the Uber Diablo DiabloWiki article.
Other Uber Monsters include: