The Black Citadel was Magtheridon's fortress in Warcraft III: The Frozen Throne. He was defeated and dethroned when Illidan and his new allies infiltrated it.
History
Map from Warcraft III: The Frozen Throne showing the Black Citadel
The Black Citadel and was a menacing structure in the Hellfire Peninsula region of Outland. Shortly after the destruction of Draenor by Ner'zhul's many portals, a pit lord named Magtheridon used the Black Citedal as a throne from which to began his rule over Outland. Magtheridon used the the portals Ner'zhul created to summon members of the Burning Legion, bolstering his armies and hold on Outland.
Illidan Stormrage and his lieutenants - Lady Vashj of the Naga, Kael'thas Sunstrider of the Blood Elves ("This group of blood elves then helped defeat Magtheridon and claimed his Black Citadel as their own."Template:Cite) and Akama of the Broken - led their followers through Outland in an effort to hide Illidan from his master, Kil'jaeden. He attempted this by destroying all portals in Outland, the access points to Twisting Nether from which the Burning Legion could enter. Thinking himself finally safe from Kil'jaeden's wrath, Illidan shifted his goal from simple self-preservation to conquest. He set out to take control of Outland from the cruel Magtheridon and rule over Outland in his place.
The top spires of Magtheridon's Black Citadel.
The Citadel, however, was heavily guarded. Magtheridon controlled a myriad of demons, including the corrupted Fel orcs that relentlessly attacked Illidan's forces. Infernal Contraptions sat silently along the walls to hurl devastating attacks on intruders. A huge abomination named The Butcher lingered in the dark recesses, waiting for prey. Six Fel Orc Blademasters were set up to guard: Bonethirst, Maim, Bloodgrin, Nera'thor, Sagra'nel and Rend. Guarding the final areas were the Master of Pain and Mistress of Torment along with an army of Burning Legion minions. In order to get through these defenses, Illidan enlisted Lady Vashj to attack the Citadel by water and Akama to infiltrate Magtheridon's inner chambers through stealth.
Illidan and his company succeeded in defeating Magtheridon and named themselves the new rulers of Outland. The victory was short lived as Kil'jaeden appeared to remind Illidan that he could not escape the far-reaching gaze of the Burning Legion, not even in the Black Citadel.
Black Citadel post Warcraft 3
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This article contains information from the Warcraft RPG which is considered non-canon.
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Illidan the Betrayer stands as the lord of Outland and rules his domain from a massive, black iron ziggurat. Ner'zhul was originally responsible for this hulking edifice, but now blood elves, naga, and illidan's few other allies roam its corridors. The naga live in underground sewers, and the upper levels teem with blood elves, traps, evil and death. Rumors tell that beneath the citadel sprawls a great prison, trapped within are heroes from previous wars, languishing in unknown solitude.Template:Cite
Speculation
Citadel vs. Temple
It is currently unclear if the Black Citadel later became the Black Temple or Hellfire Citadel, despite what has been said from 
[Blizzard] officials. More recent sources have provided further conflicts against it, and the details of what has been retconned and what is infact correct is very unclear. For a full analysis of this issue, please see Citadel Vs. Temple.