Bone Mage (Cold)
Burning Dead Mage (Fire)
Horror Mage (Lightning)
Horror Mage (Poison)
(All are spawned by the Übers)
|Super Unique Monsters||Über Baal|
|Area Level Normal||50|
|Area Level Nightmare||75|
|Area Level Hell||83|
- Note: considering that the Pandemonium Event is a Battle.net only quest and is not available in the Single player or even open Battle.net games, it is generally not considered to be a part of official lore.
"After her legions of Succubi and strongest daughter Andariel were killed by mortals who successfully defended the Worldstone, Lilith, the Queen of the Succubi and matron of all demons, swore to personally intervene. She masterminded the revival of Diablo by manifesting herself in the mortal realm and performing a ritual of Dark Summoning. This ritual, long thought to be impossible by the most powerful mortal wizards, allowed Lilith to link with Diablo across realms, strengthening him until he was finally able to once again reincarnate himself in the mortal plane. Rage and a deep yearning for revenge boiled within the eyes of the Lord of Terror, and together he and Lilith resurrected the two remaining Prime Evils as well as Duriel and Izual, two of their strongest agents.
Diablo announced his grand plan - the Three were finally strong enough to spread Pandemonium throughout Sanctuary. The Three amassed their forces in a ravaged outpost within the mortal realm - Über Tristram. It is only here that heroes can hope to best the Three once more, and save Sanctuary from certain destruction."
— Lilith's description(src)
Über Tristram has the same base structure as the regular Tristram, but has a pack-load of hellish monsters, like the rare and powerful Balrogs. There are also varieties of Unique Monsters, Champions and their minions along with the masters of the pack, the Über Prime Evils themselves. The main objective for the player(s) upon reaching Über Tristram is to find and defeat Über Baal, Über Mephisto, and Pandemonium Diablo (called so since Über Diablo already exists in-game as part of another event).
Über Mephisto has a Conviction Aura, which causes -65% to all of the player's/players' resistances, and summons Skeleton Mages and Archers everywhere, which can be a challenging task. Pandemonium Diablo is a "tank" for Über Baal. He has tons of life and summons Pit Lords which are immune to everything but Physical damage. Über Baal is pretty much the same as Hell difficulty Baal. He has a little more life though, and summons Ghoul Lords and Ghosts, which are both Physical Immune.
Once the player(s) kills them, the last one of them to die will give the player(s) the Hellfire Torch, a Unique Large Charm which gives +3 to any one of a certain class' skills, and random amounts of resistances and attributes, and the Standard of Heroes. The Standard of Heroes is a symbol that shows that the player(s) has/have completed this quest successfully.
Tips for Players
A fool-proof way to kill the Übers is to run straight up when you enter the portal. Then go around to the left and go halfway down. Then slowly walk toward the center of Tristram. This causes Diablo and Baal to charge at you. Run away to the left and top again so Mephisto doesn't instantly kill you. Then work on Diablo first because his Pit Lords can kill you pretty quickly. Baal comes next. It may take a few tries, but make sure you make your game permanent, so if you die, you can save and exit and come back. After the two easiest are dead, Mephisto is left. He is easily the hardest monster in the entire game, though there are a few different ways to kill him.
Depending on your skill as a player, the speed of your computer, and the ability of your build. The first way to kill him is to just charge in and attack like a madman. If Life Tap isn't cast within the first 10-15 seconds, you will die and have to start over again, though by going to the various magic vendors (such as Akara in Act I) you can buy a Wand with charges of Life Tap. Simply by having the wand in your second weapon set allows you to cast Life Tap before going in, making all of the three bosses trivial and eliminating the luck factor involved with other methods. The second way to kill him is to trigger him, then run into a building away from the portal, and let him follow you into it and get stuck. Then, either teleport out (if you're wearing Enigma), die, or save and exit. Then, after you're back in the game, run straight to the house and attack him. Since he is kept away from his minions, his Conviction Aura is less of a threat, though this may take a few tries.