Instructions
Uber Diablo
Wait until you see the server-wide message "Evil Walks the Earth," signaling the beginning of the Uber Diablo event. This message appears after a certain number of "Stone of Jordan" rings have been sold on the server. The number of rings is random and unpredictable.
Go to any dungeon or overworld location that spawns a "Super Unique" boss monster. The first "Super Unique" boss that spawns will be replaced by Uber Diablo. The quickest and easiest "Super Unique" spawn location is Shenk, the first boss of Act V. Go to the first waypoint in Act V and walk down a couple of screens until you see Uber Diablo.
Attack Uber Diablo. It is recommended to arm yourself with high resistance and defensive items. Diablo deals a large amount of physical and magical damage from a variety of spells and attacks.
Pandemonium Event
Defeat "The Countess" in the Black Marsh tower in Act I, "The Summoner" in the Arcane Sanctuary in Act II, and "Nihlathak" in the Halls of Vaught in Act V. Each of these bosses drops a special key in Hell difficulty. You can kill these bosses multiple times for more keys.
Go to Harrogath in Act V. Place all three keys in the Horadric Cube and combine them. One of three random portals will open to an Uber Boss. You will need at least three sets of keys to open all three portals. These portals lead to the bosses Lilith, Uber Duriel and Uber Izual. Each of these bosses drops a special item.
Combine "Diablo's Horn," "Baal's Eye" and "Mephisto's Brain" in the Horadric Cube while standing in Harrogath. This will open a portal to Uber Tristram.
Defeat Uber Baal, Pandemonium Diablo and Uber Mephisto in Uber Tristram. These bosses are much harder than the normal versions, possessing a range of new abilities and enhanced statistics.