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Well, don't know why you'd want a guide, it's rather simple.
Run hell countess over and over again in 8 player games until she drops Terror Keys. Keep running until you have 3.
Do the same with hell summoner, for 3 Hate keys.
And, guess again, same thing for hell nilathak, for 3 destruction keys.
Now the fun begins, get your strongest char (I do mean strongest, there aren't many chars who can take on uber clones), get a friend or two with their strongest char (it is possible to do alone, but very difficult)
Make a passworded hell game, go to act5 and open (on 3 different places in town) the uber portals by transmuting one of each key (you put 1 terror key, 1 hate key and 1 destruction key in your horadric cube and transmute)
Now you enter the portals one by one and kill/tp until you come accross a clone of a strong enemy (Matron Den has Lilith, an Andy clone, Forgotten Sands has Duriel, a Duri clone, Furnace of Pain has Izual, Izzy clone)
They all drop a bodypart (you'll see what those are) Put the 3 bodyparts in your horadric cube (in town) and transmute to open a final uber portal, Tristram.
If you enter here, you'll find an Uber Mephisto, Uber Baal, Uber Diablo and they all raise fast and a lot of Uber minions. When you killed the Uber Three (as they're referred to) the final kill will drop a Unique Large charm (the Hellfire Torch) and some kind of Flag (Standard of Heroes), sofar no one knows what it does, it seems to be a reward for killing them, like an ear when you kill a person in a duel.