The Pandemonium Event is a Battle.net only event introduced in Patch 1.11 of Diablo II: Lord of Destruction that is entirely optional but still fun to complete and famous for being the only source of the unique Large Charm, the Hellfire Torch.
Unlike the other Battle.net only quest, the Uber Diablo, the Pandemonium Event is quite a bit more complex and has a lot more fights with some very special super uniques specially created for the quest.
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 cannot be said to be a part of official lore. It can be said that it was created only to give high-level characters something to do.
The Keys and Organs
The initiation of the quest involves the collection of the three separate Organs that will open the portal to Chaos Tristram when transmuted in the Horadric Cube. These Organs, in turn, each require the collection of three Keys to open a portal which itself contains one Organ. Nine total keys, three of each type, are thus the minimum required to access Chaos Tristam.
The Keys are required to open the three portals that will help you collect the Organs. There are three different keys, and to open any one of the minor portals, a complete set of three different keys are required. This means that the player will have to collect, in all, three keys of each type, which totals to nine keys.
To get the keys, the player has to kill one of the agents of the Prime Evils on the mortal plane. They are:
They need to be killed in a Hell difficulty game in which they have a chance to drop the Keys of Hate, Terror and Destruction respectively along with their normal drop. The dropping of keys is quite rare, sometimes dropping only one in 10-15 runs. You can still run any one of them and continue to collect only one type of key. It is advisable to only run them for only three keys that are required for quest completion.
Once you have a complete set of three different Keys, head to Act V, Harrogath. Put them in the Horadric Cube and then transmute them. In a way, it is very similar to the Secret Cow Level, you will create a portal teleporting you to a place where you would have to fight one of either:
They will give you, when defeated, Baal's Eye, Mephisto's Brain, and Diablo's Horn respectively, the Organs that will help you create the portal to Chaos Tristram.
It is highly recommended that you collect all nine required Keys and open all the three Portals within the same game. If they are opened in separate games, there is a chance that the same portal is opened leading to a waste of three different Keys. Of course, the organs are still valuable, leaving them open to be traded for others.
Once you have all three Organs, you can to go back to Harrogath again and put them into the Horadric Cube and transmute them. You will create the portal to Uber Tristram.
Uber 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 a variety of unique monsters, champions and their minions along with the masters of the pack, the Uber Prime Evils themselves. The main objective when the player reaches Uber Tristram is to find and defeat Uber Mephisto, Uber Baal, and Pandemonium Diablo (because Uber Diablo already exists).
Über Mephisto has a level 20 Conviction Aura, which causes -125% to all of your resists, and summons Skeleton Mages and Archers everywhere, which can be challenging. Pandemonium Diablo is a "tank" for Baal, meaning he takes the damage for him. He has tons of life and summons Pit Lords which are immune to everything but Physical damage. Über Baal is mostly the same as Hell mode Baal. He has a little more life and summons Ghoul Lords and Ghosts, which are both Physical Immune.
Once you kill them, the last boss to die will give you the Hellfire Torch, a Large Charm that gives +3 to any one of a certain class' skills, and random resistances and attributes, and the, as yet unusable, Standard of Heroes. The Standard of Heroes is a symbol that shows that you have completed this quest successfully and, unfortunately, for the trouble to obtain it, does nothing else, for the moment.
The easiest way to kill all three Ubers, as they are called in game, is a Smiter, a paladin who uses the skill Smite. They are an easy build for players to create and use.
Smiters can use different types of equipment, depending on the ability to get said gear. For the most part, the gear consists of: Body Armor: Chains of Honor Rune Word or Enigma Rune Word Weapon: Grief Rune Word or Last Wish Rune Word Boots: Gore Rider Gloves: Dracul's Grasp Belt: Verdungo's Hearty Cord or String of Ears Shield: Exile Rune Word or Herald of Zakarum Helm: Harlequin Crest, Crown of Ages or Delirium Rune Word Rings: Raven Frost, Bul-Kathos' Wedding Band, and Carrion Wind Amulet: The Cat's Eye or Highlord's Wrath.
A foolproof way to kill the Ubers is to run straight up when you enter the portal. Then go around to the left and go halfway down. Then SLOWLY walk towards 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 instant kill you. Then work on Diablo first because his Pit Lords can kill you pretty quickly. Then Baal. Keep attacking until Dracul's Grasp's Life Tap is cast on them. 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 easist are dead, Mephisto is left. He is easily the hardest monster in the entire game.
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 crazy. If life tap isn't cast within the first 10-15 seconds, you will die and have to start over again. 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 your wearing Enigma, or die, or save and exit. Then after your 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 from the mage skeletons. But this may also take a few tries. Good luck, and have fun.