Uber Prime Evils
Uber Tristram is part of the endgame in Reign of Terror.
As of right now, it is some of the hardest content in the game. Along with the only way to obtain the Hellfire Torch and Annihilus Small Charm.
You will fight Uber(Harder) versions of Mephisto, Diablo, and Baal.
Each one of the three has a chance to drop either an Annihilus, or a Hellfire Torch. So when you are done, you will have a combination of the two, that totals three.
Example: 1 Torch and 2 Annihilus, or 3 Torches and 0 Annihilus
But your fight doesn't happen that fast. First you need to farm yourself three sets of Keys (3 of each, Key of Destruction, Key of Terror, and Key of Hate).
Those can be farmed by either by farming Hidden Ubers, or farming The Countess (The Forgotten Tower), The Summoner (Arcane Sanctuary), and Nithilak (Halls of Vaught).
Hidden Ubers have a 100% chance to drop key, compared to just 15% chance to get a key by farming the other option.
But if your character can't farm the Hidden Ubers, then you do have another option so don't worry!
Obtaining the Organs
After you have farmed 3 of each of the keys is when you will move on to the next step, obtaining the organs.
The first step is to have all 9 keys in your inventory and take them with you to the Horadric Device.
The Horadric Device is located in the basement of the house, right beside the waypoint in the Rogue Encampment, you enter the basement through the trap door. After you spawn in the basement, make your way to the back room where you'll see the Horadric Device in the middle of the room.
You will then use the Device to open 3 portals. You will be given the option of three different Orders of Keys, You want to make sure you use three different orders, to open three different portals at the same time.
Once the portals are open, you will enter each portal and fight the "Mini" ubers inside to obtain the organs.
The Final Battle
This next part is what you have been working for since you started your key farming. Uber Tristram.
After you have done all of the above, you will have the materials, and option to open the portal to Uber Tristram.
With the organs in your inventory, use the Horadric Device once again. This time you will get a different option. Open it up and enter the new portal that appears.
You will then spawn in Tristram, Uber Tristram. Mephisto, Diablo, and Baal all lay in wait.
The best way to be successful is to pull each boss one at a time. Look on your minimap, you'll see a big "Skull" showing where each of the 3 Ubers are.
Which ever one is closest, start moving towards him slowly until he aggros you then turn around and have him follow you away from his brothers.
After you defeat the first Uber, pull the second just like you did the first.
Lastly, take down the last Uber standing for the Victory!
You need 3 of each key to start your journey to Uber Tristram
The first option when activating the Horadric Device
The 2nd option, make sure too use 3 different key orders.
You only need 1 of each organ to gain entrance to Uber Tristram.
Your order of endgame should go like this:
This means if you have a hard time killing one, don't try the next till you can confidently kill the previous one. Farm SR if you need more gear.
Farm the Hidden Ubers for keys, its faster and you'll get a ton of other loot.
Fighting for the organs is easy, you should have no problem with those.
Pull each Uber separately, it makes the fight a piece of cake.
The Torch is a Relic, and the Annihilus is a Medal, they do not work in your inventory, they have to be equip.
The Torch and Annihilus are pretty OP, so its best to at least see if they are better then other items you have filling those slots
Tips / Tricks
Example of Annihilus Small Charm
The Pandemonium I Bosses
Some Info about Torches and Annihilus'
Example of Hellfire Torch
There is Generic and Class Specific Annihilus', so I believe you have the chance to get a +3 to a skill you want. (This may be wrong, will update if need be).
Each Diablo II Class has its own Hellfire Torch.
There is also Class Specific Trinkets, but those drop from the Hidden Ubers.