Organ Set

Includes:

1 Diablo's Horn
1 Baal's Eye
1 Mephisto's Brain

The Organs

Uber Duriel in the Forgotten Sands
Uber Izual in the Furnace of Pain
Lilith in the Matron's Den

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.

Uber Tristram

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.