Hellfire Citadel is well known to just about everyone upon first entering Outland. It is the base of operations for the Fel Orc hordes, and the first dungeon that players get to enter at level 60. The Citadel was originally built by the orcs before they first entered Azeroth, around the time they were eradicating all traces of the Draenei from the world. The building of the Citadel was orchestrated by Blackhand soon after he had been named the first Warchief of the horde being organized by Gul'dan and the Shadow council. The Citadel was to be a base of operations for the war on the Draenei, and eventually, the war on Azeroth. Here, the orcs devised new weapons and trained new recruits to fight. And it was near here, at the throne of Kil'jaeden, that the orcs drank of the blood of Mannaroth, tainting themselves and dooming their kind to the service of the Burning Legion.
The area was not always dry and dusty as it is now. It was once a thriving land, probably similar to Nagrand. But the orcs turning from the Shamanistic ways to the demonic powers of the Warlock caused the elements to abandon the land, creating the namesake, Hellfire. After the destruction of Shattrath, the orcs found fewer and fewer Draenei to kill, and it was at this time that Gul'dan received a vision about other words to conquer. The construction of the Dark Portal began. Upon completion, the armies of Fel Orcs, tainted with demon blood, marched from Hellfire Citadel to the portal and waged war with Azeroth.
The Citadel was the main staging ground for the orc's ventures into Azeroth until after the second war when the portal was sealed and the world of Draenor was destroyed from the resulting imbalance of the many portals that Ner'zhul had tried to open. Until recently, when the portal was reopened, no one had known what fate had befallen the Citadel.
During the wars, Kargath Bladefist and the orcs of the Shattered Hand clan were left in charge of the Citadel. When the Alliance attacked the Citadel, the orcs were defeated and Kargath was able to escape to Nagrand, but he could not get any reinforcements to retake the Citadel. He bided his time and was eventually rewarded when Magtheridon, a servant of Mannaroth, allowed Kargath and his clan to partake in the blood of Magtheridon, turning them into the red Fel Orcs. With this new power, Kargath was able to retake the Citadel, and he swore alliance to Magtheridon.
Illidan Stormrage, after his defeat by Arthas in Northrend, fled to Outland and took control of the Black Temple. Fearing retaliation from the Burning Legion for his failure to dispose of Arthas and the Lich King, he began building armies. Somehow he was able to over power Magtheridon, a Pit Lord of the Burning Legion, and imprison him beneath Hellfire Citadel. Kargath, probably to avoid being destroyed himself, then swore his loyalty to the new ruler of Outland, Illidan, and was put in charge of overseeing the creation of Fel Orc armies by siphoning blood from his former master within Hellfire Citadel.
When we enter Hellfire Peninsula and begin assaulting the Citadel, we are essentially fighting against Illidan's forces, and eventually killing the imprisoned Magtheridon so that he doesn't get loose and cause total havoc. It's a little bit confusing since both we and Illidan are fighting against The Burning Legion, but Illidan isn't willing, or trusting, of anyone else so we have to fight him off to. This theme seems to be in the game a lot with multiple factions fighting the same enemy, but being unwilling to join together. The Scarlet Crusade and the Argent Dawn all hate the scourge, but they have issues with each other as well. The Aldor and Scryers fight the Legion, but don't get along. The Naaru lead these fractioned groups against the Legion and against Illidan at the same time.
Thanks for reading, and have a good morning!