How Do You Spawn Uber Tristram In Diablo 2?

You need to get 9 keys, 3 each of Terror, Hate, and Destruction from The Countess, The Summoner, and Nihlathak in Hell. These will allow you to open 3 red portals, where you will fight Uber bosses to get three more special items. Putting these 3 special items in the Cube will open the portal to Uber Tristram.

Here are the details from the wiki

The first step is to gather the three keys required to open the first of the red portals. The Keys of Hate, Terror, and Destruction are required, and can only be found from certain boss monsters, and only on Hell difficulty. Get the Key of Terror from The Countess, the key of Hate from The Summoner, and the Key of Destruction from Nihlathak. It's a good idea to have 3 of each key before beginning the second stage of the Pandemonium Event, so you can do all three of the special areas in the same game. (The same area will not repeat in the same game, so you'll have all 3 ingredients for Uber Tristram.) The keys look identical, and can only be discerned between by hovering on them to see their name.

Key of Terror These are dropped by The Countess, who is located on the fifth level of the Forgotten Tower in the Black Marsh, Act One. She is commonly run for her rune drops, and will drop a Key of Terror about 8% of the time she dies on Hell difficulty.

Key of Hate The Summoner drops this one. He's found at the end of one of the four points of the Arcane Sanctuary, in Act Two. He will only drop a Key of Hate on Hell difficulty, and even then only every few games.

Key of Destruction Nihlathak sometimes drops this key, but only on Hell difficulty. He's found at one end of the four-pointed Halls of Vaught, the lowest of the three Halls levels reached through Anya's portal in Harrogath, in Act Five.

Opening the Red Portals

Once you have the keys, stand in Harrogath, on Hell difficulty, place one of each in your Horadric Cube and convert them. A red portal will open to one of three locations; the Matron's Den, the Forgotten Sands, or the Furnace of Pain. Which portal opens first is random, but if you do all three in the same game you will not get the same one twice. For this reason it's recommended that you have 9 keys to begin with, 3 of each type, so you can do them all in the same game. It's also recommended that you stand in different places so you'll spread out the three portals, so they don't overlap and make selecting the correct one 'uber difficult'. You'll be adding a 4th portal once you've completed these three, to Uber Tristram, so save room.

Uber Tristram

To create a red portal to Uber Tristram, place Diablo's Horn, Baal's Eye, and Mephisto's Brain in the Horadric Cube, and transmute them while standing in Harrogath in a Hell difficulty game. A red portal will open, the fourth in that area if you've done them all in the same game. It takes you to Uber Tristram, where the real fun begins.

As per the Diablo 2 Wiki:
Uber Tristram is part of the Pandemonium Special Event. This event can only be accomplished on Battle.net, on hell difficulty.

Uber Tristram
To create a red portal to Uber Tristram, place Diablo's Horn, Baal's Eye, and Mephisto's Brain in the Horadric Cube, and transmute them while standing in Harrogath in a Hell difficulty game

Obtaining items for Uber Tristram
This is a three step process. You need keys from three uniques.

Step 1:

• Key of Hate: The Summoner drops this key. He's at the end of one of the paths in the Arcane Sanctuary.
• Key of Terror: The Countess in Act 1 drops this.
• Key of Destruction: Nihlathak drops this key.
  These will ONLY drop on Hell difficulty, and you may need multiple runs to get all three keys.

Step 2:

Once you have all three, put them in the Cube, and transmute them. If you are in Harrogoth, on Hell difficulty, a red portal will open to one of three random areas:

• The Matron's Den
• The Forgotten Sands
• The Furnace of Pain

Each one has a boss at the end that will drop one of the three items you need. Be warned, these are much harder than Hell Baal. The bosses are, respectively:

• Lillith - Looks like Anariel. Beware her poison. Drops Diablo's Horn.
• Uber Duriel - Looks like Duriel, obviously. Drops Baal's Eye.
• Uber Izual - Beefed-up version of Izual. Beware his minions; they can be very nasty. Drops Mephisto's Brain.

The portal that opens is random, but combining more than one set of keys in one game will not open a portal to an area you already have open. As such, getting three of each key is recommended.

Step 3:

Once you have killed all three bosses, you have the items needed to go to Uber Tristam! Put them in the cube and transmute. The new red portal will go to Uber Tristram.