Hellfire Citadel: Magtheridon's Lair » Zones » WoW Armory Light

Magtheridon's Lair is the raid wing of the Hellfire Citadel dungeon in Outland where you fight the brutish pit lord Magtheridon himself. You can find the entrance to Magtheridon's Lair on the very bottom of the western side of the Citadel (opposite side from the other 3 entrances). It is a quick raid encounter (like Onyxia's Lair, which contains four "guard mobs" before the boss encounter, Onyxia). 

Players can get a preview of Magtheridon by doing the Blood Furnace instance. He is being held captive underneath the room where you fight Keli'dan the Breaker.