Birth of the Lich King


When Ner'zhul left the dying world of Draenor, he was immediately captured by Kil'jaeden. The demon was eager to take revenge on the orc, who had turned his back on the Blood Pact he had sworn years earlier. Ner'zhul's spirit was kept alive while his body was slowly torn apart. Entirely at Kil'jaeden's mercy, Ner'zhul agreed to serve the demon once more; exactly as the Deceiver had planned. Ner'zhul's spirit was encased in the Frozen Throne, a mystical block of crystal. Encased in the ice he felt his consciousness expand ten thousandfold, while being warped by Kil'jaeden's power. The orc that had been Ner'zhul ceased to exist, and the Lich King, a being of unfathomable power, was created.
Formation of the Scourge
The Lich King was sent to Azeroth, landing in Northrend where the ice that encased him shaped into the shape of a throne, where he would begin the formation of the undead Scourge and in the process weaken the world in preparation for the Burning Legion. This new army would not fall victim to the petty infighting that had caused the orcs to fail in conquering Azeroth. Sent to watch over him were the Dreadlords, led by Tichondrius himself. Within the Frozen Throne, the Lich King experimented with his psychic powers and enslaved the local indigenous life forms. The plague of undeath that came from the Frozen Throne transformed each of them into his undead servants. Thus using his psychic and necromantic powers he was able to conquer much of Northrend. As he devoured more and more souls he only grew in power as the individual undead under his control gave him "much needed nourishment". Thus his powers began growing at an exponential rate; a fact that the dreadlords were well aware of.
The Lich King entered a war with the kingdom of Azjol-Nerub, whose ancient inhabitants were immune to his plague of undeath. The 10-year conflict known as the War of the Spider ultimately ended with the Lich King's first major triumph (while the Nerubians were immune to the plague, their corpses could still be reanimated). The Lich King was impressed by his enemy, however, and adopted Nerubian architecture for his own, as a testament to the Spider Lords' tenacity and age.
Having by now established control over most of Northrend, the Dreadlords urged Ner'zhul to proceed with the agreed-upon plan to prepare the world for the invasion of the Burning Legion.
The Lich King then used his telepathy to reach out into Azeroth and summon any dark soul that would hear his call. Kel'Thuzad, a mage and a prominent member of Dalaran's Kirin Tor, answered his call. Kel'Thuzad was soon ensnared by the Lich King, faithfully serving him as the first of a Cult of the Damned; a cult that would worship the Lich King as a god and be taught necromancy to better aid their undead armies. (For the full story, please see Road to Damnation)
Kel'Thuzad and the Dreadlord Mal'Ganis were instructed to begin paving the way, but Ner'zhul, ever mindful of Kil'jaeden's schemes, secretly sought a way out of his prison...

Bristling with power and free to roam the world once more, Illidan set out to find his own place in the great scheme of things. However, Kil'jaeden confronted Illidan and made him an offer he could not refuse. Kil'jaeden was angered by Archimonde's defeat at Mount Hyjal, but he had greater concerns than vengeance. Sensing that his creation, the Lich King, was out of his control, Kil'jaeden ordered Illidan to destroy Ner'zhul and put an end to the undead Scourge once and for all. In exchange, Illidan would receive untold power and a true place amongst the remaining lords of the Burning Legion.
Illidan agreed and immediately set out to destroy the Frozen Throne, the icy crystal cask in which the Lich King's spirit resided. Illidan knew that he would need a mighty artifact to destroy the Frozen Throne. Using the knowledge he had gained from Gul'dan's memories, Illidan decided to seek out the Tomb of Sargeras and claim the Dark Titan's remains. Using his vast, demonic powers, he lured the serpentine naga from their dark undersea lairs. Led by the cunning witch Lady Vashj the naga helped Illidan reach the Broken Isles, where Sargeras' Tomb was rumored to be located.
With the powerful Eye of Sargeras in his possession, Illidan traveled to the former wizard-city of Dalaran. Strengthened by the city's ley energy lines, Illidan used the Eye to cast a destructive spell against the Lich King's citadel of Icecrown in distant Northrend. Illidan's attack shattered the Lich King's defenses and ruptured the very roof of the world. The Lich King had no defense against this spell, and he would have been forever vanquished that day. But at the final moment, Illidan's destructive spell was stopped when his brother Malfurion intervened, sensing that the spell was causing great damage to the world.
Civil War in the Plaguelands
Now that Ner'zhul had openly defied the will of the Legion, he knew that the wrath of Kil'jaeden and his demonic lackeys would be fierce, but, at the worst possible time, Ner'zhul was losing his magical power. When he had pushed Frostmourne from the Throne, he had caused a crack within the icy cask. Illidan's spell had caused this crack to worsen and now the Lich King's powers were rapidly seeping out like blood from an open wound. Half-way across Azeroth, residing over the unholy remains of his father's kingdom, Arthas was losing power — he got his power directly from the Frozen Throne through his blade Frostmourne, and his hold over the undead was slipping.
Ner'zhul knew that his time was short. Imprisoned within the Frozen Throne, he suspected (correctly) that Kil'jaeden would send his agents to destroy him. Desperate to save himself, he called his greatest mortal servant to his side: the death knight Prince Arthas.
Though his powers were drained by the Lich King's weakness, Arthas had been involved in a civil war in Lordaeron. Half of the standing undead forces, led by the banshee Sylvanas Windrunner, had been freed by the Lich King's sudden loss of influence and resented what they had become. Also, another group of undead was still under the control of the Legion's remaining commanders: the three dreadlords Varimathras, Detheroc and Balnazzar. Arthas, called by the Lich King, was forced to leave the Scourge in the hands of his lieutenant, Kel'Thuzad, as the war escalated throughout the Plaguelands.
Ultimately, Sylvanas and her rebel undead (known as the Forsaken) claimed the ruined capital city of Lordaeron as their own. Constructing their own bastion beneath the wrecked city, the Forsaken vowed to defeat the Scourge and drive Kel'Thuzad and his minions from the land. The Lich King was powerless to stop them.
Weakened, but determined to save his master, Arthas reached Northrend only to find Illidan's naga and blood elves waiting for him. He and his nerubian allies (in the form of Crypt Fiends, led by the fallen Spider Lord Anub'arak) raced against Illidan's forces to reach the Icecrown Glacier and defend the Frozen Throne.