From Diablo Wiki
Special Quests are quest-like events that are not part of the in-game quest menu, but that are activated by following special rituals and sequences of events. These include the Secret Cow Level, Uber Diablo, and the Pandemonium Events.
 The Secret Cow Level
This hidden bonus quest is an Easter Egg, based on the old jokes about a "Secret Cow Level" in Diablo I. It's fully playable and profitable as well as amusing. Reaching the Secret Cow Level is easy; simply transmute (in the Horadric Cube) a book of town portal + Wirt's leg, while standing in the Rogue Encampment, after having defeated Baal on that difficulty level. (The quest works in D2C as well as D2X, in classic you need merely defeat Diablo.)
See the Secret Cow Level page for more details.
 Uber Diablo
This upgraded version of Diablo is spawned by selling a number of Stones of Jordan on a particular Battle.net game server. Uber Diablo appears in the place of one SuperUnique monster in the game. Players must search him out and defeat him to receive an Annihilus charm as a reward for killing the demon. Uber Diablo is far more powerful than regular Diablo, and presents a substantial challenge for most characters. Uber Diablo is only available when playing on the Realms, and not for single player gamers.
See the Uber Diablo Quest page for more details.
 The Pandemonium Event
This series of bonus quests was added to the game in v1.11, and is meant as a very difficult challenge for very high level characters. Three keys must be gathered, one each from The Countess, The Summoner, and Nihlathak. These keys are then transmuted to create three different red portals, leading to the Matron's Den, Forgotten Sands, and Furnace of Pain. On each of those levels an uber version of one of the game's bosses will be encountered. When slain they drop one of three special items: Mephisto's Brain, Diablo's Horn, and Baal's Eye. These three items are then transmuted to create a red portal to Uber Tristram, where the ultimate battle takes place. Uber Mephisto, Uber Baal, and Pandemonium Diablo await bold adventurers, and if defeated they will drop one Hellfire Torch unique large charm.
See the Pandemonium Event page for full details on this multi-part, high-difficulty, realm-only quest.
|Quests of Diablo II - Quest Items [ ]|
|Diablo II||Lord of Destruction|