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 thousand fold, 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.
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. In 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 and Kel'Thuzad)
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...
After preparing for many long months, Kel'Thuzad and his Cult of the Damned finally struck the first blow by releasing the plague upon Lordaeron. Uther Lightbringer and his fellow paladins investigated the infected regions in the hope of finding a way to stop the plague. Despite their efforts, the plague continued to spread and many of Lordaeron's northern most settlements were consumed utterly.
As the ranks of the undead swept across Lordaeron, King Terenas' only son, Prince Arthas Menethil, took up the fight against the Scourge. As was the Lich King's intention all along, Arthas succeeded in killing Kel'Thuzad, but even so, the undead ranks swelled with every soldier that fell defending the land. Frustrated and stymied by the seemingly unstoppable enemy, Arthas took increasingly extreme steps to drive them out - he eventually ordered the slaughter of everyone in Stratholme, which had been infected by the plague, to prevent Mal'ganis from adding them to the army of the dead. Finally Arthas's comrades warned him that he was losing his hold on his humanity.
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Arthas' fear and resolve proved to be his ultimate undoing. He tracked the plague's source to Northrend, intending to end its threat forever. There he stumbled across his long-time friend, Muradin Bronzebeard, brother to the dwarven King Magni, and the dwarf led him to a legendary weapon they hoped would help them combat the Scourge. Instead, Prince Arthas himself fell prey to the Lich King's tremendous power. Believing that it would help him save his people, Arthas took up the cursed runeblade, Frostmourne. Though the sword did grant him great power, the cost was high: Muradin lay dead, and Arthas began to lose his soul, transformed into the first and greatest of the Lich King's Death Knights. Arthas finally exacted revenge upon Mal'ganis, removing one of the Lich King's more dangerous jailers and completing the unholy transformation. With his soul cast aside and his sanity shattered, Arthas led the Scourge against his own kingdom - Lordaeron. Arthas murdered his own father, King Terenas, and crushed Lordaeron under the Lich King's iron heel.With Arthas as his champion, Ner'zhul spread the Plague throughout Lordaeron. What remained of the Order of the Silver Hand struck back, but even mighty Uther fell to the Death Knight's power. On orders from Tichondrius, Arthas took the Scourge north, to the high elven kingdom of Quel'Thalas, to resurrect the Summoner of Archimonde - Kel'Thuzad. The high elves never stood a chance, and their capital, Silvermoon, was ravaged - their millenia-aged Sunwell warped and used to resurrect Kel'Thuzad as a lich. Thus, both masters were appeased: the Lich King's most loyal worshiper was returned, and the Summoner was unleashed.
Having progressed too far into the Legion's plan (and guarded too closely by Tichondrius) to back out now, Ner'zhul's minions laid siege to Dalaran, reclaiming a spellbook of Medivh... which contained the incantations needed for Kel'Thuzad to summon Archimonde.
Finally, Archimonde was summoned in Dalaran, and he immediately gave control of the Scourge to Tichondrius and the Dreadlords. But the Lich King was not done yet. Archimonde may have removed Ner'zhul's control over the undead, but in his eagerness for vengeance against the Kaldorei, he forgot to return the Frozen Throne to Kil'jaeden. Thus, the Lich King remained at large.
During the Legion's invasion of Ashenvale, Illidan Stormrage was released from his barrow prison after ten thousand years of captivity. Realizing Illidan's addiction to magic, and having utilized the Skull of Gul'dan himself years earlier, the Lich King dispatched Arthas to Kalimdor. There, Arthas covertly told Illidan about the powers of the Skull of Gul'dan. Unable to resist such power, Illidan took up the skull and harnessed its vast energies. By doing so, Illidan developed demonic features and vastly magnified power. Illidan, exactly as the Lich King had planned, then proceeded to kill Tichondrius and liberate Felwood.
Without Tichondrius's support team, Archimonde's overconfident ascent of Mount Hyjal led to the unexpected: his annihilation.
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.
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, and his power had begun to seep 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.