"I... am... UNLEASHED!"
Magtheridon was a pit lord, and a former commander of the Burning Legion, and former Lord of Outland.
Following the destruction of Draenor as a result of Ner'zhul's reckless creation of portals, Kil'jaeden sent Magtheridon to the shattered ruins of the world to claim it for the Burning Legion. Making use of the dimensional gateways Ner'zhul had opened, he was able to quickly rally an army of demons and claimed the shattered world for himself. He rallied many of the surviving orcish tribes to his banner, and corrupting them further, causing them to transform into Fel Orcs. He claimed the ruined draenic city of Karabor, now known as the Black Temple and/or Black Citadel, to serve as his seat of power. With the bulk of the surviving orcs under his control, and a steady flow of reinforcements coming from the dimensional gateways, it seemed impossible that anyone would contest his control of Outland. His only opposition, after all, was nothing but a few scattered tribes of draenei, ogres, what few orcs escaped corruption, and maybe some flightless arakkoa.
"The might of Magtheridon cannot be denied! I am the Lord of Outland! Those who dispute my rule will soon regret their folly."
The situation changed considerably with the arrival of Illidan and his allies. With help from
Prince Kael'thas' blood elves, and
Lady Vashj's naga, and Akama's draenei survivors, Illidan was able to successfully overthrow the pit lord. After sealing the dimensional portals, and cutting off his reinforcements, the Illidari attacked the Black Temple, overcoming Magtheridon's defences and defeating the him in combat. He was presumed dead, but as it turned out Illidan found a use for the defeated pit lord.
Rather than killing Magtheridon, and potentially risking having the demon's spirit simply escape back into the Nether, Illidan instead imprisoned the pit lord beneath the ruins of Hellfire Citadel. Recognizing the usefulness of the Fel orcs Magtheridon had commanded, Illidan decided to use the pit lord to produce more them, and create a new Fel Horde, albiet one loyal to him instead of Magtheridon and the Burning Legion.
Things seemed to be going well at the newly refurbished Hellfire Citadel. The remnants of the long lost Alliance Expedition at Honor Hold put up a bit of a fuss, but they were hopelessly outnumbered and there was certainly no chance of them getting any sort of reinforcements from Azeroth. At least, that was the case until
Lord Kazzak managed to reopen the Dark Portal, allowing the New Horde to establish themselves in the region, and letting the Alliance regain contact with their lost expedition. With fresh adventurers arriving through the portal daily, they soon stormed Hellfire Citadel, killing
Keli'dan the Breaker, Magtheridon's chief jailor, and weakening the integrity of his prison.
While Keli'dan's death nearly resulted in Magtheridon breaking free of his prison, however the remaing warlocks of the Fel Horde were ultimately able to keep him contained. That is, until the naaru,
A'dal, tasked adventurers with destroying the pit lord, and ending the threat of Illidan's Fel Horde once and for all. Adventurers did so, destroying the pit lord, passing A'dal's trials, and earning themselves the title of Champion of the Naaru. His head was brought to Honor Hold or Thrallmar, and presented to
Danath Trollbane or
Nazgrel, both of whom were happy to put it impale it on a spike and put it on display for all to see.
"This defeat is not only a blow to Illidan, <name>. The Legion too reels... A pit lord commander has been defeated."
Magtheridon's fate is unknown. As Outland is floating within the Twisting Nether, he is likely well and truly dead. As such, presumed dead, until proven otherwise.