Hellfire Citadel

Hellfire Citadel was the base of operations of the Fel Horde during the War in Outland, and is a nearly impenetrable bastion that served as the headquarters of the Orcish Horde throughout the First and Second Wars, and was originally built to serve as Warchief Blackhand's fortress. Having believed to be abandoned following the end of the Second War, the citadel was reclaimed by the Fel Horde under Warchief Kargath Bladefist, who made his home there along with his forces until their defeat. The citadel was composed of three key areas: The ramparts, the area known as the "Blood Furnace", where orcs were corrupted into the Fel Horde's service, and the inner halls. A chamber below the furnace was also used to keep the pit lord Magtheridon captive and siphon his blood for use in creating more fel orcs.

Alternate Universe
Hellfire Citadel is the headquarters of the Iron Horde, recently completed in the center of the Tanaan Jungle. The name was originally suggested by the late Garrosh Hellscream to his father Grommash. It is now used as a stronghold by Gul'dan and the Burning Legion after he corrupted it with his fel magics.