The Lich King

Birth of the Lich King

When Ner'zhul attempted to escape Draenor, he was immediately apprehended by the demon lord Kil'jaeden, who enacted the blood pact that Ner'zhul had sworn many years earlier. Kil'jaeden ravaged and destroyed his body, but kept his spirit alive and encased it in a block of ice from the Twisting Nether. Kil'jaeden granted him increased power, including that over death itself. Ner'zhul ceased to exist, and the Lich King was created.

Formation of the Scourge

The Lich King was sent to Azeroth through the Great Dark Beyond, landing in Northrend where the ice that encased him formed into the shape of a throne, The Frozen Throne. Here, 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 earlier. 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 and kept a good eye on him.
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 The "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 then 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 Lordaeron 

The Lich King seen within the Frozen Throne.

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. Prince Arthas Menethil and Lady Jaina Proudmoore along with Captain Falric began Lordaeron's search for answers on the new threat. Lordaeron's northern most settlements were consumed utterly by the Plague.
As the ranks of the undead swept across Lordaeron, King Terenas' only son, Prince Arthas Menethil, took up the fight against the undead. 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. This most likely included innocent slaughter to prevent Mal'Ganis from adding the citizens to the army of the dead, and strike at Mal'Ganis himself. Arthas failed to trap Mal'Ganis, who escaped to Northrend. 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 by killing its supposed leader Mal'Ganis. 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 started The Scourging of Lordaeron with the murder of his own father, King Terenas Menethil II, and crushed Lordaeron with his newfound unholy strength.

Path of the Damned

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 was 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. Before the Summoning, Kel'Thuzad had been reading through the spellbook and noted to Arthas that Medivh's knowledge of demons alone was staggering and that he was far more powerful that anyone had ever realized.
Finally, Archimonde was summoned outside 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 Damned

The Lich King imprisoned within the Frozen Throne.

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 - and 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 as well — his powers came directly from the Frozen Throne, through his blade Frostmourne, and his hold over the undead was also 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 King 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. Consuming the old sewers beneath the wrecked Capital 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 Triumphant

Arthas fighting Illidan at the Frozen Throne.

"Now...we are one!"

Arthas, with Anub'arak's help, battled his way through their forces until faced with Prince Kael'thas, who challenged him. Kael'thas used the fact that Jaina now hated Arthas to make him hesitate, and fought with his father's reforged runeblade, Felo'melorn (Flamestrike). Flamestrike clashed against Frostmourne but in the end Arthas forced Kael'thas to flee, leaving the path open. He then proceeded to magically activate the four Icecrown obelisks around the glacier, opening the doors to the Frozen Throne.
However, Illidan was waiting for him. An intense battle commenced in which Illidan displayed his newfound demonic powers and nearly defeated Arthas. Until, coming in for the finishing blow, he inadvertently left himself open and Arthas quickly took advantage of it, slicing open the demon hunter's chest. Illidan collapsed, grievously wounded. Arthas then turned towards the open doors of Icecrown, leaving Illidan on the ground instead of finishing him. Rather, before he walked away, Arthas warned Illidan to leave Azeroth and never return.
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, 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 and throughout the chamber, "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.

Illidan The Betrayer

Illidan, also known as the Betrayer, was the twin brother of Malfurion Stormrage. He was a practitioner of Highborne magic. In his youth, he attempted to master the druidic forces as his brother had, but the sorcery called to him in a way that the magic of the land did not. Unlike his brother, Illidan was born with amber eyes; at the time, this was regarded by the elves as a sign of a great destiny— however, this actually indicated inherent druidic potential. When Malfurion and Tyrande had found their destiny, however, Illidan was still searching for his. Though not a Highborne himself, he became the personal caster of the military leader, Ravencrest.
When Sargeras' invasion of Azeroth had begun and Azshara's treachery became known, Malfurion convinced Illidan to leave his queen. Illidan followed his brother. However, as Cenarius and the dragons entered the battle, Malfurion came to understand that their adversaries were too powerful to fall in combat. To end the invasion, he plotted the destruction of the Well of Eternity. The idea appalled Illidan. The Well was the source of his magic— and likely of the elves' immortality, as well —and its loss was a price far too dear for him to pay. In addition, the night elf found that he increasingly admired the powers of the Burning Legion, seeing a magical purity that underlay their chaotic behavior. Where the night elves struggled to maintain their ground, the Burning Legion's numbers did not seem to permanently diminish. The satyr Xavius seized upon his doubts, exploiting his confusion to plant seeds of distrust into Illidan's mind, which made Illidan turn to seek the power the Burning Legion was using so he could become stronger. While, in his mind, this was to help defeat the Burning Legion, he actually helped them by giving Sargeras the Demon Soul to make the opening for the portal.
Illidan had strong feelings for Tyrande Whisperwind, a novice priestess in the Sisters of Elune. Illidan so desired to impress Tyrande that he often acted without thinking, particularly in magic; he never realized that these displays weren't quite what the priestess was looking for in a mate. But while Illidan struggled to win her heart, none of them realized that the battle was over soon after it had begun; Tyrande had chosen Malfurion almost from the beginning. Xavius knew of this and used his power to darken Illidan's thoughts, convincing him that if Malfurion were to die, Illidan would no longer have a rival for Tyrande's love. Finally, the sight of Tyrande in the arms of his brother, Malfurion, shattered his final ties to the defenders.
Illidan, with a new plan spurred into his mind, journeyed to Zin-Azshari. There, he feigned allegiance to Azshara and Mannoroth. Illidan's plan was to obtain the Demon Soul, an artifact of great power created by Deathwing also known as Neltharion the Earth-Warder, which had the ability to close the portal which was allowing the demons to enter Kalimdor. However, to put this plan in action, Illidan had to gain more power. Illidan was eventually brought before Sargeras himself, who quickly discovered the night elf's plan to obtain the Demon Soul for the Legion. Sargeras was pleased with this plan and gave Illidan a "gift" in return for his allegiance. Illidan's eyes were burned out by Sargeras himself, despite still being beyond the portal and orbs of mystic fire set in their place that allowed Illidan to see all forms of magic and arcane tattoos covered his body. Azshara was fascinated by the "new" Illidan (who was wary of her advances), but remained cautious, sending Captain Varo'then to accompany Illidan in his search for the Demon Soul.

Illidan from "The Sundering".

After the Great Sundering, Illidan, who had filled seven vials with water from the Well of Eternity, scaled the peaks of Mount Hyjal, where he found a small, tranquil lake. There, he poured the contents of three of the vials into the waters. The chaotic energies quickly manifested, tainting the lake and twisting it into a new Well of Eternity. Illidan's joy was short-lived, however, when his brother, Malfurion, Tyrande and the rest of the kaldorei leadership discovered him- and all were horrified at what he'd done. Unable to accept that his brother had committed such treachery, Malfurion tried again to explain to Illidan the folly of his ways. The magic, he insisted, was chaotic by nature and it could only bring about destruction so long as it existed. Illidan refused to listen, so enraptured by the magic's power that his brother seemed to him an unknowing fool. Illidan claimed that magic would be needed should the Burning Legion ever return.
The lack of remorse shook Malfurion to his core and he raged at his brother, understanding now that Illidan was lost forever to the magic's sway. He ordered him imprisoned deep below Hyjal in a jail kept far from sight and mind.

As Illidan Stormrage may have looked in his earlier years

Release from prison

After being released.

Illidan languished for 10,000 years in a lightless prison. Califax, a Keeper of the Grove and a contingent of night elves (including Maiev Shadowsong) kept constant guard over the Betrayer. Release came at the unexpected hands of Tyrande, who slaughtered the night elf guards in the hopes of using Illidan against the Legion, which had returned to Azeroth at the hands of the Scourge. With his love for Tyrande undampened through the millennia of confinement, Illidan agreed to help. He swore to throw back the Legion and to depart from the night elves forever.

Illidan confronts Arthas.

Malfurion opposed Tyrande's decision, thinking the release of Illidan a catastrophic mistake. Frustrated that his brother remained unchanged and eager to prove to him that the demons had no hold over him, Illidan left Malfurion behind, leading a force of night elves into Felwood to hunt down the Legion. While in Felwood, he encountered Arthas, champion of the Lich King, and they engaged in combat. The fighters were equally matched and the combatants reached an impasse. Illidan stopped the duel and demanded to know why Arthas had tracked him. Arthas responded openly, telling Illidan about the Skull of Gul'dan, the demonic artifact that had corrupted Felwood. He explained that if the artifact were destroyed, the corruption of the forests would halt. To ensure that Illidan would take the bait, Arthas spared no detail of the Skull's power, adding that his master knew of Illidan's thirst for power. Although Illidan did not trust Arthas, he nevertheless sought the Skull and its power out.
A great Demon Gate defended the Skull as Illidan and his forces had to fight mightily to gain access to the artifact. Driven by necessity and influenced by the belief that with the increased power, he could at last redeem himself in the eyes of Tyrande, he shattered the demonic seal and used the Skull's powers for his own. Strength he found, but peril greater than power came to him at once. The battle for control of the Skull came with a heavy price and the artifact's powers changed him. Transformed into a demon, Illidan wrapped himself in shadow and utterly destroyed Tichondrius and his demonic forces. But with victory came devastation. Tyrande and Malfurion sensed the demonic power lurking within Illidan and turned away with disgust and despair. Malfurion chastised his brother, convinced that Illidan had traded his soul for more power. Enraged, Malfurion banished his brother from the forests.

Illidan absorbed the powers of the Skull of Gul'dan.

Illidan became a black demon after consuming the power of the Skull of Gul'dan.

Illidan defeating Tichondrius in their battle.

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Feeling his sacrifice and effort were unappreciated, Illidan muttered "So be it... brother" and left the night elf lands.

Allegiances

Illidan is visited by Kil'jaeden

After the Legion's defeat, Illidan was visited by Kil'jaeden, who, while noting Illidan's sketchy track record with the Legion, offered him one final chance to serve them. He told Illidan to seek out the Frozen Throne and destroy it. Ner'zhul had grown far too powerful for Kil'jaeden to control and Illidan was to remove him from the equation in exchange for more power and magic than he could dream of. Kil'jaeden granted him the Orb of Kil'jaeden to aid him in this task.
By harnessing Gul'dan's enchanted skull, Illidan came to possess the warlock's memories and a plan came to mind, but he would need allies to help him. He decided to enlist the aid of some old friends.
Illidan called the naga to the surface. The naga, formerly the quel'dorei servitors of Azshara, were eager to take vengeance upon the night elves and the other land-walking races that had been spared from The Great Sundering. Azshara sent her handmaiden, Lady Vashj, to establish contact; the naga, respecting Illidan's magical talent, accepted the alliance and Vashj eventually came to lead those who would be known as "Illidan's Naga". But Illidan still had the troublesome warden, Maiev Shadowsong, to deal with, as she had persistently chased him all over Kalimdor. Illidan then allied with a group of satyrs, known as Illidan's Servitors.^[11][12] Then he ran to the port of Nendis with his naga and satyr minions protecting the way up behind him. Maiev killed the furbolgs and defeated his forces, but when she came to the port, Illidan hijacked a boat and set sail, while a cadre of naga stayed behind to scuttle the ships and ruin all hopes of following him.^[11]
With the naga swimming in his wake, Illidan came ashore on the Broken Isles— the ruined remains of Suramar (ironically, the same city where Illidan grew up) which Gul'dan and his warlocks had raised twenty years before and the home of the Tomb of Sargeras.

Confrontation on the Broken Isles

Illidan calling the Naga from the depths of the sea.

Despite their best efforts, Illidan's servants had not been successful in destroying the boats. Maiev and the Watchers arrived on the Broken Isles shortly after Illidan and the two forces battled across the watery terrain. Illidan reached the tomb and Maiev was quick to follow. With Gul'dan's knowledge, Illidan quickly traversed the Tomb and came to the chamber which contained the Eye of Sargeras. Maiev came upon him as he and Vashj were activating the powerful artifact and, as vengeance for her imprisoning him for 10,000 years, Illidan used the Eye to bring the Tomb down around her and then quickly escaped through the underwater passages used by the naga. Though he killed Naisha and all the other Watchers within the Tomb, Maiev escaped with her magical abilities. On the surface, Illidan and Maiev battled for supremacy as the Warden sent out a runner to gather reinforcements from Ashenvale.

Illidan, after receiving the Orb of Kil'jaeden.

Malfurion and Tyrande arrived on the Broken Isles with reinforcements as Maiev's base was being overrun. When they struck back, Illidan's forces crumbled, but he and his retainers fled before they were seriously harmed. Tyrande pursued them out of the base and Illidan ensnared her to protect himself, warning her not to interfere. He fled across the sea once again. During this battle, Tyrande finally revealed why she had spurned Illidan: too drunk with his rising magical and political power, he had forgotten his own inner strength. Malfurion, despite his increases in power, held on to that strength within him. Armed with this knowledge, Illidan finally came to grips with his feelings.
Illidan landed on the shore of Lordaeron and quickly made his way through Silverpine Forest to get to Dalaran, where Illidan began to use the Eye of Sargeras to break apart the polar ice cap and destroy Icecrown and the Frozen Throne. But he was interrupted by Maiev and Malfurion, and the spell failed. Malfurion had felt Illidan's spell tearing the land apart and concluded that he was a danger to the world and had to be stopped. Illidan, who was entangled by his brother, called Malfurion a fool as he was destroying the Lich King, their common enemy. Malfurion was furious at him for causing the loss of Tyrande, who had been, according to Maiev, killed. Illidan was heartbroken that the woman he loved was dead, but Prince Kael'thas, the night elves' newest ally, thought that perhaps it was premature to presume her dead. Kael'thas explained that Tyrande had not been "torn apart" by the undead, as Maiev had told Malfurion, but had instead fallen into a river and been swept away by the currents. Malfurion immediately arrested the Warden and promptly went in search of Tyrande. Illidan begged to be allowed to help his brother track down the beloved priestess. Illidan and his personal guard of naga found Tyrande under heavy attack from a massive undead force.
Illidan and his naga battled their way through the undead until they reached her. Tyrande was taken aback by Illidan's service to her, and when he delivered her safely to Malfurion, she was astonished. Malfurion told Illidan he was free to go on the condition that he never threaten the night elves again. Illidan, wishing for an end to the conflict with his brother, agreed.

Escape to Outland

Illidan in Outland.

After Malfurion let him go, Illidan created a portal to Outland and fled immediately, pursued by Maiev. Now that he had failed to destroy Ner'zhul, he knew that Kil'jaeden's wrath would not spare him, so he wanted to find a world where he could remain free from Kil'jaeden. He felt that Outland, the ruined remains of Draenor, was just such a place.
Illidan was chased down on the shattered world until he was captured by Maiev and the Watchers, and imprisoned once again. But he was saved by Kael and Vashj. Illidan accepted the allegiance of the blood elves, and made Kael his second-in-command. The Sin'dorei, coupled with the naga, would be very invaluable to his plans. Illidan continued his original plan - to rid Outland of demonic influence so that he could stay out of Kil'jaeden's grasp. To do this, they laid siege to the Black Temple of Magtheridon, the Pit Lord who had taken control of the world. But first, Illidan systematically shut down his dimensional gates to stem the flow of reinforcements. Eventually, they succeeded.
When they came to the Black Temple, Illidan was approached by Akama of the Broken, who pledged the allegiance of his race. Akama's broken felt indebted to Illidan and his army for aiding them in their fight against the fel orcs of Magtheridon, who had laid siege to their village with the intent of slaughtering them all. They laid siege to the Black Temple and destroyed Magtheridon's defenses, and then defeated the Pit Lord in battle himself. Magtheridon noted wryly that Illidan had great power, and asked if the Legion had sent him as a test. Illidan laughed, saying that he was not a test, but a replacement, and imprisoned Magtheridon beneath Hellfire Citadel and kept sealed in place by the fel orc jailor Keli'dan the Breaker.

The March on Icecrown

Illidan clashing with Arthas.

Illidan is defeated by Arthas.

As Illidan rallied the forces of Outland under a new banner, a storm of fire and smoke descended upon the Black Temple, and Kil'jaeden appeared in all his unholy glory. Chastising Illidan for his foolhardy attempts to flee his wrath, Illidan quickly claimed that he had been gathering his forces for a second attack on the Frozen Throne. Kil'jaeden, seeing that Illidan's new comrades "showed some promise", agreed to give Illidan one more chance to appease the demons.
Illidan, Vashj, and Kael laid siege to Northrend and battled Anub'arak's forces as they trudged through the snow towards the Icecrown glacier. But Ner'zhul, knowing he would be overrun if he didn't do something, called Arthas to Northrend to complete the plan he had set in motion so many months before. Finally, Illidan's forces arrived at Icecrown as Arthas and Anub'arak dug their way out of Azjol-Nerub, and the two factions squared off in a titanic battle as they tried to gain control of the four mystical obelisks surrounding the glacier. After a long battle, and control going back and forth between the two enemies, Arthas activated all obelisks, and the doors to the Frozen Throne were opened.
But Illidan was not finished yet. Meeting Arthas at the glacier's base, the two warriors engaged in single combat. After a few minutes of exchanging blows, however, Arthas overcame Illidan's defenses and cut him down. Illidan Stormrage fell in the snow, dead, or so it seemed. He was badly wounded, but not killed. Arthas was quite aware of this, and before taking his leave, warned Illidan to leave Azeroth and never return. After it became clear for Vashj and Kael that they could not destroy the Frozen Throne, they retreated, eventually back to Outland, taking Illidan with them.
Note that the in-game animation between Illidan and Arthas was originally planned to be a video cinematic. Time constraints, however, forced the scene to be done inside the game. The developers have said that their greatest regret about this change is that many assumed Illidan died, when the video would have revealed that Illidan survived to watch Arthas's transformation.