The Kil'jaeden's war against the Lich King was a conflict fought primarily between the factions of the Burning Legion and the Undead Scourge. Its events were chronicled in Warcraft III: The Frozen Throne.
At the end of the Third War, the Great Alliance was able to defeat Archimonde at the Battle of Mount Hyjal. While Archimonde was dead, the aftermath and legacy of the invasion was far from over. While the Horde under Warchief Thrall and the humans under Jaina Proudmoore established their respective counties, the Night Elves decided to return back to their homelands, to not only repair the damage to Kalimdor, but to also accept that they were now mortal beings.
Kil'jaeden, knowing Ner'zhul's betrayal and aid to the Night Elves, decided to eliminate him first. Finding the recently exiled Illidan Stormrage, and promising him power beyond imagination, Kil'jaeden gave Illidan the Orb of Kil'jaeden, and the task of destroying the Lich King. Looking into Gul'dan's memories, and finding out about the Tomb of Sargeras, Illidan decided to use the Titan's power to destroy the Lich King. He would later receive assistance from the Highborne-turned Naga in escaping Kalimdor to proceed with his plan.
Eye of Sargeras
The warden, Maiev Shadowsong, once Illidan's jailor, was eager to recapture her prisoner, believing that he had become more unstable thanks to The Skull of Gul'dan. Accompanied by Naisha, she initially pursued Illidan to the end of Kalimdor, where she followed him to the Broken Isles. Establishing a base and learning of the island's history from Drak'Thul, Maiev discovers that Illidan is planning to use the powers of the fallen Titan, Sargeras, to receive even more power. Fighting her way through the demons and guardians of the tomb, Maiev arrives to see Illidan claim the Eye of Sargeras for himself, and he uses the newfound power to bury the tomb, along with Maiev and the watchers that joined her. While Maiev was able to escape, Naisha and the rest of the watchers perished under the tomb.
Now on the disadvantage, the warden manages to send a runner to inform both Malfurion Stormrage and Tyrande Whisperwind of Illidan's recent actions. Believing that letting him go was now a mistake, the pair arrive on the Broken Isles, and manage to assist Maiev in forcing back Illidan and the Naga. The trio chase Illidan all the way to Lordaeron, where Malfurion temporarily parts ways with the two to commune with the land of Lordaeron. While separated, Maiev and Tyrande encounter Prince Kael'thas Sunstrider of the elves of Quel'thalas. He informs them of the formation of the Blood Elves and the destruction of his home country. Bringing supplies for the Alliance, the Night Elves decide to aid Kael'thas, in exchange for his help in hunting Illidan. During the movement of the caravan, a large undead army swarms the elves, causing Tyrande to have a stand-off against them at a bridge to give time for Kael'thas and Maiev to leave. Though she is able to stop the advance, the bridge she fights on collapses, and the priestess is swept by the river. Kael initially attempts to rescue Tyrande, but relents on Maiev's insistence that he aid her hunt.
Having finished speaking with the trees and finding out of Illidan's plot of using the Eye of Sargeras to melt the ice caps of Northrend, Malfurion returns to inform Maiev and Tyrande of the plan, but finds the latter missing. Maiev then proceeds to lie that Tyrande was killed fighting the undead, and the two, soon accompanied by Kael, face off against Illidan. Stopping his ritual, Illidan reasons his attempt to stop the common enemy of the Lich King, but Malfurion retaliates that Illidan's recklessness had caused Tyrande's death. Interjecting into the conversation, Kael tells Malfurion that Tyrande was simply swept by the river current, and that it was premature to declare her dead. Realizing the deception, Malfurion immediately restrains Maiev, and allows Illidan to aid him in their attempt to rescue the priestess. Kael, on the other hand, parts ways with the Night Elves in order to join the remaining Alliance forces in Lordaeron under Lord Garithos.
With the brothers working together for the first time in ages, Malfurion and Illidan find Tyrande barely holding out against a massive Undead Army. Allowing himself to hold the undead army back, Malfurion leaves Illidan with the task of reaching and saving Tyrande because of how the Naga are able to reach her faster through the river. Not betraying his brother's trust, Illidan returns with Tyrande, and the two brothers make amends. Knowing that Kil'jaeden would likely be angered at his failure to destroy the Lich King as well as getting sidetracked, Illidan decides to leave Lordaeron through a portal. Maiev, having broken free, led her remaining Watchers into the portal in pursuit. Despite Tyrande's attempts to dissuade Maiev and to explain that Illidan had atoned for his crimes, the priestess is stopped by Malfurion, who states that Maiev has now become the embodiment of vengeance and is utterly consumed for the hunt. The pair soon return back to Kalimdor.
Joining up with the Alliance forces, Kael met Lord Garithos, who tasked Kael with the repair of the observatories over the remains of Dalaran. Unhappy with both the task and the commander's racist remarks, Kael begrudgingly accepts the task nonetheless. During the mission, he encounters Lady Vashj, previously affiliated with Illidan. She provides boats for him upon the discovery that the nearby Goblin Shipyard had been destroyed by the undead. Accomplishing the task, Kael is once again insulted by Garithos for having accepted the aid of the Naga to complete it. Later Kael is ordered to fight and destroy nearby encampments of the undead, but most of his forces were recalled to Garithos' own.
Left with no choice, Kael once again accepted the aid of the Naga, and was able to not only prevent the destruction of his brethren, but also accomplish the task. The two later converse, and Kael admits that the remaining elves have become power-hungry thanks to the loss of the Sunwell. Soon a Spell breaker informs the two that Garithos had arrived, and Vashj and her Naga quickly leave, but not before being spotted. Having grown incensed with the elves for accepting the aid of the Naga, Garithos proceeds lock up Kael and the remaining blood elves in the Violet Hold with the intent of executing all of them.
Unexpectedly, Vashj sneaks in through the sewers, and assists Kael in freeing the remaining blood elves. Once all of his men were assembled, Vashj opened a portal to Outland, and Kael ordered his men to cross immediately. During the process, Garithos attempted to destroy the portal by using explosives, though the elven forces were able to reconstruct old towers of Dalaran to aid in their escape. Having escaped to Outland, Vashj tells Kael that the person that can aid him is none other than Illidan. As the two traverse Outland, they soon spot that Maiev has managed to capture Illidan and has locked him in a cart. Attacking her en-route, and securing Illidan, the pair release Illidan, and he states his plan on escaping from Kil'jaeden's wrath. Planning to first cleanse Outland of Kil'jaeden's agents, he moves to take Outland from the Pit Lord Magtheridon.
War for Outland
- Main article: War for Outland
Plaguelands Civil War
- Main article: Plaguelands Civil War
Race to Icecrown
Having reached Northrend, Arthas is greeted by Kael'Thas after destroying a scouting encampment, and he declares that both he and Illidan will stop Arthas at all costs. Receiving information of a shortcut from Anub'arak, Arthas kills the blue dragon, Sapphiron, and resurrects it as a massive Frost Wyvern that helps them plow through the Blood Elf and Naga defense. Surprised to not see the forces he left behind, Anub'arak and Arthas soon find that the dwarves of Muradin Bronzebeard had regrouped under Baelgun Flamebeard and had since taken refuge in the Shadow Web Caverns. Defeating the dwarves, the undead soon run into the Forgotten Ones, and kill a Faceless One to escape the Caverns and reach the Icecrown. Although Illidan, Kael and Vashj had already established their own bases, Arthas and Anub'arak were able to secure enough time for Arthas to reactivate the four obelisks of Icecrown. In a final bid, Illidan duels Arthas, but is wounded and defeated, eliminating the final obstacle between Arthas and the Lich King.
Illidan's failure to defeat the Lich King allowed Arthas to unquestionably become one of the most powerful beings on the world. Though he initially had issues in order to become the dominant personality, he was able to banish his human side and suppress Ner'zhul's spirit. As the Lich King, Arthas would kill many and perform multiple inhumane acts to lure unsuspecting heroes that would dare to challenge him, so that he would kill them and raise them as his greatest champions.
Illidan never fully recovered from this defeat, and he became paranoid upon returning to Outland. Illidan's subsequent breakdown eventually caused Kael'thas to abandon Illidan in favor of serving Kil'jaeden instead. Seeing Illidan's insanity, Akama used this opportunity to free Maiev, and the two, along with a group of heroes, defeated Illidan and ended his control over the Black Citadel.