The 1.11 patch added to the game a so called "Pandemonium Event" available ONLY on the realms, in hell difficulty. To trigger the event, you must collect items called keys. There are three types. The Countess drops the Key of Terror, the false Summoner drops the Key of Hate, and Nihlathak drops the Key of Destruction.

When you have collected three keys, go anywhere in the town of Harrogath in Act 5 and transmute these three keys together in the Horadric Cube. One of three portals will open, to the Furnace of Pain, the Matron's Den, or the Forgotten Sands, in which you will fight Über-Izual, an Andariel-like creature called Lilith and Über Duriel respectively. Each of these bosses drops one of the Prime Evil's relics, Lilith drops "Diablo's Horn", Über Duriel drops Baal's Eye, and Über Izual drops Mephisto's Brain.

When you have collected all three relics (a.k.a. body parts) you must go back to Harrogath, and transmute the three relics. This will open a portal to Über Tristram, where you will fight improved versions of the Prime Evils all at once - the Übers. Prepare for the hardest fight of your D2 career. The last of the Prime Evils you kill will drop a large charm called the Hellfire Torch. It gives 3+ to the skill levels of a random character class, 10-20 to all resistances and attributes, a chance to cast Diablo's Firestorm on striking, 8+ to light radius and Hydra charges.