You gather the keys by killing 3 Unique monsters in Hell: The Countess (A1), The Summoner (A2) & Nihlathak (A5). (Large Charm with x3 random class skills, +10-20 to All Resistances/Attributes and 5% ctc lvl 10 Firestorm on attack) You gather keys to open portals to fight Uber Andariel, Uber Duriel & Uber Izual to obtain the Hellfire Torch.
The Pandemonium Event: ( Now Available in Single Player)