The Pandemonium Event
The Pandemonium Event is an end-game challenge, added in patch 1.11 and played in Hell only. Its finale, Uber Tristram, grants the 
Hellfire Torch, a unique large charm. Here's how it works:
The Three Portals
The event starts with three keys, each dropped in Hell only by a super unique or act boss. Every portal takes one of each key, so opening all three takes three sets.
Dark StalkerTower Cellar Level 5
Opening a portal
Transmute one of each key in the 
Horadric Cube in Harrogath, in Hell:
Key of Terror, dropped by The Countess in Tower Cellar Level 5
Key of Hate, dropped by The Summoner in Arcane Sanctuary
Key of Destruction, dropped by Nihlathak in Halls of Vaught
The portal leads to Matron's Den, the Forgotten Sands or the Furnace of Pain at random; each opens once per game.
Each portal's area has a boss of its own, Lilith, Uber Duriel and Uber Izual, which drops one of the three organs that open Uber Tristram. As each portal opens only once per game, one game gives at most one of each organ.
Uber Tristram
Opening the portal
Transmute the three organs in the Horadric Cube in Harrogath, in Hell:
Diablo's Horn, dropped by Lilith in Matron's Den
Baal's Eye, dropped by Uber Duriel in Forgotten Sands
Mephisto's Brain, dropped by Uber Izual in Furnace of Pain
In Uber Tristram you face Uber Mephisto, Uber Diablo and Uber Baal at the same time. They are far stronger than their normal selves, regenerate quickly and are immune to Prevent Monster Heal, and keep calling in more monsters during the fight. Like Lilith, Uber Duriel and Uber Izual, each of them always spawns Extra Fast, along with other Monster Modifiers (see each Uber's page).
DemonPandemonium EventUber Tristram
DemonPandemonium EventUber Tristram
DemonPandemonium EventUber TristramRewards
When the last of the three Ubers dies, it drops:
- the Hellfire Torch, with +3 to the skills of a random class
- a
Standard of Heroes for every player in the game
