Kil'jaeden (pronounced Kill-Jay-Den) is a powerful Eredar Demon Lord and is the acting leader of the Burning Legion. Elected twenty-five thousand years ago as the chief lieutenant of Sargeras, he was ranked above the demon lord Archimonde. While Archimonde had led the Legion's armies, Kil'jaeden's agenda was to assimilate every race possible into the folds of the Burning Legion. Since the apparent banishment of Sargeras, he has assumed the role of supreme commander.
He is also the creator of the The Lich King.
Destroyer's offer and the flight of Velen
Twenty-five millennia ago, the world of Argus was the home of the wondrous eredar civilization, a race of natural magic-wielders. Kil'jaeden, respected for his intelligence, a genius among a race of geniuses, quickly rose to become one of the three most prominent leaders of the eredar, along with his colleague Archimonde and best friend, Velen.
Kil'jaeden had loved Velen as a brother. Eredar society was shattered when these three leaders were contacted by Sargeras the Destroyer. Sargeras offered a deal: in exchange for their loyalty, the eredar would gain untold power and prestige. Kil'jaeden and Archimonde readily accepted, though Velen held back.
As the eredar were preparing to receive their new powers, Kil'jaeden learned that Velen and his followers had sided with Sargeras's enemies, the and were in the process of fleeing Argus. Enraged by this "betrayal" Kil'jaeden loosed his new powers and minions on the "Exiled Ones", or draenei, and relentlessly pursued them across the cosmos for five thousand years... but Velen was as cunning as the Deceiver, and the draenei soon vanished.
Orcs, the oath, and the Deceiver's revenge
Following the failure of the invasion of Azeroth, some fifteen thousand years after accepting Sargeras's offer, Kil'jaeden was ordered to find a mortal army, that could be used to invade worlds en masse. Kil'jaeden
prowled the Nether for millennia for the perfect soldiers. Then, one day, Kil'jaeden's favored servant, Talgath, stumbled upon a world that showed signs of the draenei's magic. Though they initially assumed it was just another one of the planets the draenei briefly stopped upon, further investigations quickly discovered that the draenei had actually settled there, even going so far as to give it an Eredun name: Draenor, meaning "Exiles' Refuge".
Sharing the "Exiles' Refuge" with his nemesis were a race of shamanistic beings called "orcs". Intrigued by their potential, Kil'jaeden realized that they were exactly what he was looking for.
Going straight to the Horde's most respected leader, and using the guise of the orcs' sacred ancestors, Kil'jaeden tricked the elder shaman, Ner'zhul, into thinking that the draenei were plotting against the orcs, and subtly introduced warlock magic and bloodlust to the clans.
Soon, the clans began to launch successful skirmishes and raids against the draenei, believing it to be their ancestors' will that they be exterminated. However, plans faltered when Ner'zhul travelled to Oshu'gun, was shown the truth about his new patron, and attempted to undo his actions. Kil'jaeden, however, was already aware of Ner'zhul's actions and stripped the elderly shaman of his power, investing the greatest and darkest of warlock magicks into Ner'zhul's betrayer: his erstwhile apprentice, Gul'dan. Eventually, as the Elemental Spirits refused to grant their powers to the shaman, these warlock powers would overtake the orcs.
Gul'dan's indomitable lust for power and his selfish personality made him the perfect pawn for the demon. After proving the orcs' ability to fight the draenei, Mannoroth's blood was given to the orc chieftains to drink, and they degraded into a horrible bloodlust. As both a test of his new minions and as revenge to his wayward brethren, Kil'jaeden ordered Gul'dan to finish the slaughter of the draenei in one climactic battle at Shattrath City. The attack was a brutal massacre with much of Shattrath being destroyed. To all appearances, Kil'jaeden's pawns had finally triumphed over the Exiled Ones.
Kil’jaeden, like the rest of the eredar warlocks, felt the presence of Medivh when he began his astral roaming. Kil'jaeden, relatively pleased with the Horde's destruction of the Draenei, took this as a sign that Sargeras required his energies elsewhere. The Eredar demon retreated from Draenor, abandoning the Horde. Aware that the time had come to unfold his stratagem, Medivh manipulated Gul'dan into opening the Dark Portal and sending the Horde deep into Azeroth. He promised Gul'dan a home for his people, and plenty of inhabitants to slaughter to satisfy their blood lust. Medivh's agendas were nearly fulfilled as the Horde made their way into Azeroth, spreading their dark, demonic energies wherever they went. They heralded the destruction that would wipe out any resistance that could be offered to defend against the Legion, destroying the Kingdom of Azeroth, almost completely subjugating Khaz Modan, and enslaving the mighty Alexstrasza.
However, two events occurred that would inevitably doom the orcs' invasion to failure. The first was the assassination of Medivh, which in turn caused the great spirit of the Dark Titan in him to be banished to the Abyss. The second, ironically, was caused by Gul'dan's lust for power, the same trait that had originally been used to corrupt the orcs. Gul'dan facilitated the Horde's fall in the Second War by seeking out the Tomb of Sargeras which, as told to him by Sargeras himself through Medivh, he believed would give him all the powers held by the Dark Titan before his demise eight hundred years before. As a result of his actions, Orgrim Doomhammer lost nearly half his standing forces and the orcs lost their one chance of wiping out the last bastion of human power for good. Given enough time to regather and reorganize, the humans followed with a fierce counter-attack that ultimately won them the war. The orcs had failed.
The Lich King
Kil'jaeden, infuriated, knew that the time had come to take matters into his own hands. Recalling the Nathrezim's experiments with undead during the War of the Ancients, and realizing that it was free will that led to the downfall of the orcs (the betrayals of Doomhammer, the Stormreaver and the Laughing Skull clans), he formulated a most sinister plan: Why not create an army of enslaved undead, under the control of a single cunning mind cowed in fear to the Legion?
Calling in an unpaid vendetta, the Deceiver found Ner’zhul within the Twisting Nether, and, seeking vengeance against the shaman's defiance, tortured the orc for an impossibly long time, destroying his body piece by piece until finally, nothing remained but the shaman's tortured soul. Kil'jaeden offered the remnant of the soul a choice: remain in eternal pain in the eredar torture dimension, or become the ruler of an army of undeath. Ner'zhul, predictably, chose the latter, and was transformed into the Lich King. Recalling Ner'zhul's previous duplicity and Gul'dan's failure, Kil'jaeden took no chances whatsoever. He refused to give the Lich King a body, instead sealing specially forged armor, along with the Lich King's soul, into the Frozen Throne. Just in case Ner'zhul wasn't daunted by immobility, he dispatched the Dreadlords to make sure he stayed on task.
The Lich King was to succeed where the orcs had failed: His undead servants would wipe out any potential resistance, and raise the unfortunate fallen as undead legions. The Scourge was just that, and wiped out Quel'Thalas and much of Lordaeron to prepare for the Legion's invasion. By the time Archimonde entered Azeroth, the remaining forces of Lordaeron were no match for them.
However, the Lich King, aware of the eredar's hatred for the immortal elves, betrayed the Legion by breaking the pact he had forged with Kil'jaeden which bound him to the latter's will. In informing Illidan of the Skull of Gul'dan through his pawn Arthas, he facilitated the death of one of the Legion's greatest tacticians, Tichondrius, and dealt the first blow against the Legion that led to the ultimate fall of their forces in the Battle of Mount Hyjal.