ok basically this is how it goes down.

hell countess has a small chance to drop the Key of Terror. only she can drop it, and that chances that she will are fairly low (i think it was like 1 in 30?).

The Hell Summoner drops the Key of Hate, and Nihlathak drops the Key of Destruction.

You take those three keys, and trasmute them while in town in hell act 5. this opens a portal to one of three places (the names slip my mind). there is a large map in which u want to go find the uber and kill him. upon killin him/her they drop an organ. after killing all 3 different ubers and getting each organ (eye, brain, horn) you trasmute them in the cube, again in town act5 hell. this opens a portal to Uber Tristram. This is the hardest part of the whole ordeal. in the portal there is an overpowered diablo, mephisto and baal all together. if you kill all three of them, they drop a Torch, and Standards of Hero's.

if you havent heard of a torth, it is a very handy large charm that adds these stats.

25% chance to cast lvl 10 firestorm on striking
3 to Char skills (can be barb, pally, sorc, ama, sin, necro, or druid skills)
+10-20 (varies) All Attributes
+10-20 (varies) All Resistances
8 to light radius
lvl 30 hydra (10/10 charges)

a standard of heros is nothing but a token showing u have killed the ubers.