The elder shaman Ner'zhul was once among the greatest spiritual leaders of the orcs. Deceived by the demon Kil'jaeden, he set in motion the events that led to the orcs' corruption and the creation of the bloodthirsty Horde. Yet ultimately Ner'zhul refused to give his people fully to the demonic ranks of the Burning Legion.
The demon lord Kil'jaeden punished Ner'zhul for his defiance, destroying his aging body and torturing his spirit. The demon then offered Ner'zhul a final ultimatum: serve the Legion unconditionally or suffer eternal agony. With little other choice, Ner'zhul pledged to obey Kil'jaeden and was reborn as a terrifying and vastly powerful agent of the Legion – the Lich King.
Ner'zhul's spirit was magically shackled to a suit of ancient armor and bound to the mighty runeblade Frostmourne. To ensure Ner'zhul's obedience, Kil'jaeden sealed the armor and blade within a specially crafted block of ice collected from the far reaches of the Twisting Nether. This frozen crystal was then cast into the ripe and unsuspecting world of Azeroth, settling in the desolate, snowy wastes of Northrend.
Among the abilities bestowed upon the Lich King was lordship over death itself. From within his Frozen Throne, Ner'zhul summoned a number of undead to serve him and tested his army against the nerubians of Azjol-Nerub and their mighty spiderlord, Anub'arak. Although the War of the Spider raged for years, many nerubians who fell in battle quickly became bound to the Lich King's iron will. Anub'arak himself was eventually ambushed and slain, rising again to join Ner'zhul's ranks as a fearsome crypt lord.
The Lich King appeared to be faithfully serving his master, but he had in fact hatched a cunning and subversive plan. To that end, he created a small fissure in his prison, pushed Frostmourne through it, and directed his minions to carry the runeblade away from the Frozen Throne. Ner'zhul intended to use the sword as bait for a mighty champion: a loyal subject who would free him and serve as a vessel for his restless spirit. While Frostmourne was positioned to serve its future purpose, the Lich King dutifully proceeded with his demon master's true agenda...
Since arriving on Azeroth, the Lich King had been formulating an insidious plague of undeath, a terrifying disease meant to annihilate humanity and create an army faithful to the Burning Legion. To expedite the spread of this contagion, the Lich King recruited a powerful ally in the ambitious mage Kel'Thuzad, a senior member of Dalaran's ruling council.
Under the Lich King's watchful gaze, Kel'Thuzad created the Cult of the Damned, a group of humans to whom he promised social equality and eternal life on Azeroth. The cultists spread the plague throughout Lordaeron's northern villages, amassing a host of mindless undead. Kel'Thuzad looked upon the growing army and named it the Scourge, for at the Lich King's whim it would surely scour humanity from the face of the world.
The archmage Antonidas suspected that the undead plague was magical in origin, and so he sent the sorceress Jaina Proudmoore to the northlands to investigate. With her went Prince Arthas Menethil, only son of King Terenas. Jaina and Arthas tracked down and killed the Lich King's servant, Kel'Thuzad, but the necromancer's death failed to halt the rise of the Scourge. As the battle against the undead wore on, the prince's faith and patience began to wane.
Joined by the legendary paladin Uther the Lightbringer, Arthas and Jaina nevertheless arrived at the gates of Stratholme too late to stop the distribution of plague-ridden grain. Arthas realized that the innocent villagers would inevitably join the growing ranks of undead. He ordered Uther to purge the town and slaughter the civilians before they could become minions of the Scourge. When Uther refused, Arthas accused the veteran knight of treason and disbanded the Order of the Silver Hand. Uther and most of his remaining cavalry left the village in disgust, and Jaina, horrified by Arthas' decision, abandoned the prince as well.
Despite the loss of his supporting forces, Arthas carried out his plan, putting the city's innocents to the blade and burning every remaining structure to the ground. Something in Arthas snapped that day, and as he walked away from Stratholme, he left much of his humanity in the flaming ruins.
The prince then dedicated himself to stopping the Scourge at all costs. In time he traced the source of the plague to Northrend. With a heavy heart and steadfast determination, the prince set out for the snowy roof of the world. In Northrend Arthas was surprised to find an old friend, the dwarf Muradin Bronzebeard. Muradin had been searching for a blade renowned to have legendary powers: the blade called Frostmourne. Arthas resolved to seek out the enchanted blade and wield it against the Scourge. However, at Uther's urging, King Terenas soon recalled Arthas and his troops. Before the expedition could set sail for home, Arthas secretly hired indigenous mercenaries to burn the ships. Leading his troops to the ships, Arthas feigned surprise and ordered his bitterly disappointed soldiers to hunt down and kill the mercenaries.
Arthas then scoured the desolate wastes for what he believed was the key to his people's salvation. In time Muradin and Arthas discovered Frostmourne and read the foreboding inscription on its pedestal. The inscription warned that Frostmourne's wielder would indeed gain power eternal, but at a cost: "Just as the blade rends flesh, so must power scar the spirit." Despite the warning and Muradin's protests, Arthas swore that he would pay any price to possess the blade.
At the prince's reckless urging, Frostmourne freed itself from the ice in which it was encased, in the process taking Muradin's life. Arthas seized the blade, which destroyed what little remained of his humanity. The prince could not have guessed that the Lich King was the blade's true master, nor could he have known that in addition to Kil'jaeden's enchantments, Frostmourne possessed an ability that had been bestowed by Ner'zhul: the ability to steal the souls of the living.
With Frostmourne in hand and a growing darkness in his heart, Arthas wandered into the frozen wastes to answer the call of his new master...
All of Lordaeron rejoiced at Prince Arthas' triumphant return from Northrend, but their elation was not to last. Not long after Arthas knelt before King Terenas, the former paladin thrust Frostmourne into his father's heart. At the Lich King's command, the fallen prince went on to murder his mentor, Uther the Lightbringer, and claim Kel'Thuzad's remains. When Arthas had finished, Capital City, formerly one of humanity's crown jewels, had been reduced to a silent wasteland of death and despair.
The death knight Arthas then marched his ever-growing army of the undead into the forests of Quel'Thalas, slaughtering the elves in his path as he smashed through the gates of Silvermoon. Arthas laid claim to the legendary Sunwell, using its powers to raise Kel'Thuzad as a lich and leaving the high elven lands utterly devastated in his wake.
Kel'Thuzad in turn opened a gateway to usher in Archimonde, a powerful demon lord of the Burning Legion. Upon his arrival Archimonde declared that the Lich King's usefulness had reached an end. While Archimonde pursued the Legion's goals, Arthas traveled to the continent of Kalimdor. There he met and advised Illidan Stormrage, a powerful demon hunter whose interests aligned with those of the Lich King.
The Third War ended with Archimonde's demise, and Legion forces on Azeroth were scattered in the wake of his death. Arthas returned to Lordaeron, where he was assailed by painful visions that he learned were a sign of the Lich King's weakening power. The death knight journeyed back to Northrend and was confronted by an army of elves: survivors of the Scourge invasion of Quel'Thalas. The elves had joined forces with Illidan Stormrage in order to march on the Frozen Throne. The crypt lord Anub'arak arrived and guided Arthas into a series of ancient tunnels that allowed the pair to reach the Lich King quickly.
Arthas emerged from the subterranean caverns to face one final challenger barring his path: Illidan Stormrage, who had become a demon. Illidan engaged the death knight in single combat, Frostmourne clashing against Illidan's Twin Blades of Azzinoth. Ultimately the demon was left bloodied in the snow as Arthas advanced, undeterred by the voices of the past echoing in his head, warning him against the action he was about to take.
Some say that Arthas was still in control of his own actions when he ascended the steps of the Frozen Throne; others claim that he had been the Lich King's creature ever since taking up Frostmourne. Whatever the case, a single voice stood out in the death knight's mind as he reached the runic armor encased in ice. "Return the blade," commanded the Lich King. "Complete the circle. Release me from this prison!" With a roar, Arthas brought Frostmourne down, shattering the Frozen Throne. He then donned Ner'zhul's helm, sealing his union with the Lich King.
The Lich King waits now, scheming to lure adventurers down the same dark path that Arthas tread. Frostmourne hungers for the souls of both the brave and the foolish, and somewhere Ner'zhul's voice still echoes within the helm of the Lich King.
"Now, we are one."