The Lich King was an entity created by Kil'jaeden the Deceiver from the spirit of the orcish shaman Ner'zhul. Initially trapped within the Frozen Throne, the Lich King eventually merged with the human Arthas Menethil and they became the new Lich King. He is the creator of the Scourge and rules over it telepathically from his throne atop the Icecrown Glacier.
Menethil's time as the Lich King came to an end after the Ashen Verdict's seige of Icecrown Citadel, the death of the second king of the Scourge is accredited to Highlord Tirion Fordring and the players that followed him. Arthas' last words are documented as being "I see only darkness before me."
Heralded by the words of Terenas Menethil's spirit ("There must always be a Lich King.") Bolvar Fordragon, doomed by the events at the Warathgate and the seething torture that Menethil had inflicted upon him, took up the crown of domination as his own and became the third and current king of the Scourge. This was hidden from the world, to keep them free from the tyranny of the scourge and the horrifying truth.
The current Lich King sits upon the Frozen Throne, encased in ice, locked in a subconscious battle between the soul of Bolvar and that of Ner'Zhul as Arthas did long ago...
Birth of the Lich KingEdit
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 ScourgeEdit
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...
Scourge of LordaeronEdit
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 that 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.
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 (or so Arthas believed) 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.
Path of the DamnedEdit
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 millennia-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 night elves, 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.
Legacy of the DamnedEdit
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 PlaguelandsEdit
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.
The Lich King TriumphantEdit
After a short, but intense battle, Illidan inadvertently left himself open and Arthas took advantage of it, slicing open the demon hunter's chest with Frostmourne. Illidan collapsed in the snow, grievously wounded, as Arthas turned towards the open doors of Icecrown.
Arthas entered the hollow glacier and beheld a winding pinnacle chained to the ice. As he strode up the stairs towards his destiny, the voices of those he had forsaken flooded through his mind. First he heard again Uther warning him "If we allow our passions to turn to bloodlust, then we will become as vile as the orcs." Then he heard again more angry remarks from Uther and Muradin Bronzebeard, as well as his own responses to them, yet he ignored them, continuing his ascent. Finally, he reached the pinnacle and before him he saw an icy cask, within which was a suit of armour, arranged as if seated on a massive throne. Now only two voices spoke to him: Medivh's previous warning to Jaina and the rasping whisper of Ner’zhul:
"Your young prince will find only death in the cold north."
"Return the blade... complete the circle... release me from this prison!"
With a great cry of strength, Arthas brought the might of Frostmourne to bear against the Lich King's icy prison and with a haunting scream, the Frozen Throne exploded, and shards of the crystal scattered on the ground. With Ner’zhul’s thorny helm at his feet, Arthas leaned forward, picked it up and then placed the unimaginably powerful artifact on his head.
"Now," Ner'zhul's voice echoed within his mind, "we are one!"
In that moment, Ner'zhul and Arthas' spirits fused into a single mighty being, just as the Lich King had always planned. And thus one of the most powerful entities on Azeroth was born.
According to Matthias Lehner, the Lich King removed his own heart some time after his battle with Illidan, believing his last strand to humanity, or anything that made him at all mortal made him weak. After removing his heart the Lich King returned to the Frozen Throne where he sat dormant for several years, waiting and dreaming, while his minions constructed the saronite fortress Icecrown Citadel around the Frozen Throne.
When the Lich King awoke, he journeyed to Sindragosa's Fall where he also raised the ancient dragon Sindragosa, the first consort to Malygos, as a frost wyrm before as he watched his massive undead army prepare for war.
Wrath of the Lich KingEdit
The Lich King recently unleashed a new plague upon both Kalimdor and the Eastern Kingdoms. When it was cured, he sent abominations and frost wyrms to attack Orgrimmar and Stormwind. Following these assaults, both the Alliance and the Horde set sail for Northrend to do battle with the Scourge.
These attacks were intended to lure powerful heroes to Northrend, where the Lich King seems to corrupt them and use them against their own people, in a reflection of Arthas' own journey.