The Third War was a conflict between the mortal races of Azeroth and the Burning Legion. Unlike the previous wars, the Third War took place in the Eastern Kingdoms and Kalimdor. In the war, the ancient night elves were reintroduced to the world. Its events were chronicled in Warcraft III: Reign of Chaos.
After their defeat in the Second War, the Old Horde's Warchief, Ner'zhul was captured by Kil'jaeden for his failure. As punishment, the Deceiver destroyed Ner'zhul's body, and tortured his spirit until he agreed to serve the Legion once again. His soul encased in a suit of armor which was then bound to the Frozen Throne, Kil'jaeden hurled his creation towards Azeroth where it smashed into Icecrown Glacier. Thus the first major player of the Third War, the Lich King was born. Once on Azeroth, the Lich King took control of Northrend and he had the necromancer Kel'Thuzad take the Plague of Undeath to Lordaeron.
Before the start of the war, the prophet Medivh came to the young orcish Warchief Thrall in a dream. Medivh told him that he must rally the newly-reformed shamanistic Horde and move them across the sea to Kalimdor in order to escape the shadow that was coming to consume the land. Trusting the prophet's words, and wishing to find a new home for his people, Thrall listened to his call, fought his way through Lordaeron's troops and sailed west.
Scourge of Lordaeron
Disturbance in the north
A rumor had started to travel around Lordaeron telling how a mysterious plague had gripped the northern lands, as the majority of Lordaeron's attention was focused on the Horde's flight and rebelling orc clans. When Medivh appeared before King Terenas telling him just as he told Thrall: to save his people, he must travel west, Terenas told him that he would defend his kingdom. To do so, he sent his own son Arthas Menethil, who was dealing with the rebelling orcs with Sir Uther the Lightbringer north to investigate. Jaina Proudmoore from the Kirin Tor was sent from Dalaran to aid in the investigation.
Cult of the Damned
After joining Jaina Proudmoore, prince Arthas discovered that Andorhal's granaries were wrought with a plague that killed all who are infected (within a period of time) after they had encountered Kel'Thuzad in the town of Brill. Since Andorhal was the main distribution point for grain in the area, this meant that many other towns were in danger. As they fought their way through the infected city of Andorhal they bore witness to a unique devastation like Azeroth had never seen. They hunted down and attacked Kel'Thuzad for his crimes against the people. The necromancer told Arthas that he served a greater power, a dreadlord named Mal'Ganis, and that he was responsible for the Scourge. Enraged, Arthas vowed he would kill Mal'Ganis and they traveled to Stratholme to confront him after slaying the corrupted wizard.
Rise of the Scourge
While in Hearthglen, Arthas and Jaina's forces were attacked by legions of undead. Arthas had Jaina search for Uther while he and his forces remained to protect the town from the Scourge. It was then he learned the truth behind the plague. The plague didn't just kill those infected, it turned them into the undead, and thus, the Scourge. Before his army was defeated, Uther and his knights arrived and saved the town. Not long after this battle, Arthas was met by Medivh, who warned the prince to travel west to Kalimdor. Like his father, Arthas refused, saying his place was with his people. Jaina believed the prophet's words but Arthas would not be swayed to leave his kingdom. They continued to Stratholme to deal with the threat of the Scourge.
Culling of Stratholme
Upon arriving at Stratholme, they discovered that the grain had already been distributed amongst the townsfolk. Arthas, not wanting to deal with an entire army of undead, ordered Uther and his paladins to destroy the town. Uther was appalled by this, saying he wouldn't follow the order even if Arthas was the king. For his defiance, Arthas disbanded the Knights of the Silver Hand and called all those still loyal to Lordaeron to come with him. Uther took his followers and left, and Jaina quickly followed. Abandoned by his teacher and friend, Arthas continued to Stratholme to slaughter the town.
Upon arrival, Arthas met Mal'Ganis himself. The two had a race of sorts for the lives of the town. Arthas wished to destroy them, Mal'Ganis wished to corrupt them. When the town was in ruin, Arthas demanded a final showdown with Mal'Ganis. Mal'Ganis, however, slipped away, vowing to meet him in Northrend.
After the Culling, Jaina and Uther returned to Stratholme to bury the dead. Jaina was met by Medivh, who told her Arthas would only die if he went to Northrend. The prophet then told Jaina, as he had told everyone else, that she must travel west to Kalimdor, a request she answered to.
Rise of the death knight
After arriving in Northrend, Arthas discovered the brother of the dwarven king Magni Bronzebeard, Muradin. Upon arriving at Northrend, Muradin's expedition was attacked by the undead and was cut off from the rest of the world. He had come to Northrend to obtain the runeblade Frostmourne. When word from Lordaeron called Arthas's men back home, the prince used mercenaries to burn their ships before turning his forces on the mercenaries. With no way to get home, he told his men that their only way home was to get the Frostmourne. When he finally found the blade, he and Muradin learned that the blade was cursed. Arthas accepted this curse and took the blade, though Muradin was believed to be killed in the process (he was knocked out with terrible amnesia). With the Frostmourne, Arthas lead an assault on the Scourge base where Mal'Ganis was stationed and killed the dreadlord. With the cause of his madness dead, Arthas fled to the wilderness of Northrend leaving his men to fend for themselves.
Months later, Arthas returned to Lordaeron to get his just rewards. When he greeted Terenas, he took up the cursed sword and, without a moment of consideration, slew his father. Arthas then named himself King of Lordaeron and gave the land to the Scourge. Weeks later, Arthas, now a death knight under the orders of the Lich King, was given the task to bring the dead necromancer Kel'Thuzad back from the dead. To do so, Arthas had to take his remains to a place of magical power. Arthas killed his former mentor Uther for the urn he was carrying. The urn carried the ashes of his father, to which Arthas simply disposed of and placed Kel'Thuzad's remains inside. The only magical nexus with the necessary power was the Sunwell deep inside Silvermoon, capital of the high elves. To revive Kel'Thuzad, the Scourge would have to burn Quel'Thalas.
Fall of Quel'Thalas
Upon reaching the borders of Quel'Thalas, the Ranger-General Sylvanas Windrunner became a quick thorn in the death knight's plan. Despite the mighty elf gates, the destruction of an important bridge, and many well trained rangers, the Scourge plowed their way through Quel'Thalas' defenses on their way to Silvermoon, and from there the Isle of Quel'Danas and the Sunwell. Before Sylvanas could warn Silvermoon of the Scourge invasion, her camp was raided and she was taken prisoner. Arthas tortured her and, before giving her the peace of death, he used his power to turn her into the first banshee, making her a slave to the Scourge. With its defenses destroyed, the Scourge entered Silvermoon and slaughtered all the high elves in the city. When Arthas reached the Sunwell, he placed Kel'Thuzad's remains in the enchanted waters, thus turning him into a lich and corrupting the Sunwell. With his business in the area done, Arthas had the Scourge kill the remaining high elves of Quel'Thalas.
With Kel'Thuzad revived, they had to do their job for the dreadlords. The Scourge traveled south to Dalaran to obtain the Book of Medivh and use it to summon Archimonde himself to Azeroth. The Kirin'Tor used their most powerful magics, but failed to prevent the Scourge from stealing the spellbook. With the book, Kel'Thuzad summoned Archimonde and the Burning Legion to Azeroth. Archimonde's first order of business when entering the world was to destroy Dalaran with his great power. From this point and for the rest of the war, Archimonde, the Field Commander of the Legion, led the invasion of Azeroth himself, and planned to destroy Nordrassil, the World Tree at Mount Hyjal.
Flight to Kalimdor
Shortly after the culling, before Arthas had killed Terenas, Jaina Proudmoore brought thousands with her to Kalimdor following Medivh's request.
Having fled the Eastern Kingdoms, the Horde was scattered while sailing past the Maelstrom. Some of the ships landed in a chain of islands near the Maelstorm where they met the Darkspear tribe. Here the orcs aided the trolls, then led them on an escape to Kalimdor.
In Kalimdor, Grom Hellscream, chieftain of the prominent Warsong Clan, had landed with his clan. Before they could regroup with Thrall, they ran into Jaina's soldiers and battles soon began.
When the rest of the Horde reached and regrouped on Kalimdor, Grom directly defied Thrall's orders by attacking Jaina Proudmoore's encampment after giving in to his instinctual bloodlust. As a punishment he was sent north to the massive Ashenvale Forest where he would collect lumber to build a capital for the orcs.
Meanwhile, Thrall began his search for the rest of the Horde, and eventually came across the tauren, and their leader Cairne Bloodhoof. After helping Cairne repel invading centaurs, Cairne told Thrall of an oracle up in the Stonetalon Mountains who could aid the Horde in their journey.
Upon reaching the Stonetalon Mountains, Thrall and Jaina met the prophet who called doom long before the Scourge of Lordaeron, Medivh. Medivh asked that the Horde and the Alliance work together, forgetting their old hatreds to battle a new, more powerful enemy.
Grom Hellscream and his clan started collecting lumber as they had been ordered to. However, by doing so, they enraged the night elves. After battling the night elves, their demigod Cenarius stepped in personally to deal with the Warsong Clan.
After fighting a lost battle, the orcs realized that they couldn't kill Cenarius by conventional means. Cenarius saw the orcs as demon spawn and therefore only their total eradication would satisfy him. To defeat them, Grom and the followers drank from a mysterious fountain that empowered them, not knowing that it had been defiled by the blood of the pit lord Mannoroth. Energized and turned into fel orcs, the Warsong successfully attacked and killed Cenarius. With the orcs once again doing the bidding of the Burning Legion, Mannoroth stormed up to Grom and revealed it was his blood that empowered the orcs. Despite their victory, they were once again under demon control, and thus the control of the Burning Legion.
To help their new allies, Jaina created a magical Soul Gem. Thrall would need to capture the mighty Grom within it, and take the gem to a prepared magical circle to free his spirit of corruption. Standing between the humans and free orcs, however, was the entire Warsong clan, fortified with scores of demons that waited in ambush and mighty infernals that rained from the sky. Despite his woes of killing many of the Warsong clan, Thrall captured Grom and brought him back for a ritual of cleansing. Along with Thrall's shamans, Jaina, and Jaina's best priests and sorcerers, they managed to pull the demonic taint out of Grom. Thrall and Grom then challenged Mannoroth, and though Thrall was defeated, Grom managed to deliver a killing blow, destroying Mannoroth and freeing the orcs at the cost of his own life. Thus Grom redeemed himself for bringing them into the legion.
Return of the night elves
When it became evident that the Burning Legion had returned, Tyrande led her Sentinels into the trees, fleeing a wave of undead which had destroyed a human and orc camp. As they fled, they were cornered by dreadlords and Archimonde. Tyrande believed that with the Defilers' return the only way to defeat the Burning Legion was to use the forces of old, the sleeping druids and their leader, Malfurion Stormrage, her old love. After waking Malfurion, the next order of business was to awaken the rest of the Druids of the Talon and the Druids of the Claw.
In their quest to free the druids, they discovered the prison that held the old demon hunter, Illidan Stormrage. Tyrande believed that the Betrayer, as the night elves grew to call him, could be a powerful ally against the Burning Legion and left Malfurion to awaken the druids while she freed Illidan. Malfurion continued on to awaken the druids, who had grown feral after almost ten millennia in the caves. However, the Horn of Cenarius freed them of their madness.
Tyrande continued her quest to free Illidan, fighting her way through the warden's guards. Tyrande placed her trust in Illidan, believing his power would help them against the Burning Legion. Malfurion, however, still distrusted his reckless brother. Eager to prove himself, Illidan vowed he would show his brother the demons had no hold over him and he led a portion of the night elf army to Felwood. Shortly after, the death knight Arthas found and battled with Illidan. As the two were evenly matched, there was no reason for Arthas to engage Illidan. Illidan demanded that Arthas reveal the reason for his presence in Felwood. Arthas told him of a powerful artifact: the Skull of Gul'dan. The skull was responsible for the corruption of the night elves' beloved forest. Arthas played to Illidan's thirst for power, mentioning that the skull held immense power, and Illidan took the bait willingly.
After fighting his way through the demons, Illidan reached and consumed the power of the Skull. With its power added to his own, he slew the leader of the dreadlords, Tichondrius, and helped seal the Legion's defeat. When Malfurion and Tyrande found the new demon-night elf that was now Illidan, Malfurion banished Illidan for his demonic corruption, claiming that he was "no brother of his". A very resentful Illidan tore his way through the dead trees and was not heard of until the coming of the naga.
Battle of Mount Hyjal
After Illidan's banishment, Medivh called Malfurion and Tyrande to a secret grove where he had Thrall and Jaina meet them. Medivh revealed that he was the one who helped Burning Legion create the Dark Portal and allowed the Horde to invade Azeroth. After he was slain for his crimes, he had to return to right his wrongs and protect Azeroth from the Legion's return. Medivh asked that the mortal races unite to defeat the Legion, as their combined power was their only hope.
To prepare for the demons' attack, three camps were built around Mount Hyjal. Malfurion devised a plan to use the power of the World Tree itself to destroy Archimonde. As he went back up the summit to make the necessary preparations for Archimonde, Jaina and Thrall used whatever night elves Tyrande could spare to fortify their bases. Archimonde tore his way through ranks of human knights, elven sentinels, and orcish warriors and smashed his way through the human castle and orc fortress. Jaina was able to teleport most of the surviving troops. Before Thrall escaped death, he managed to hurt Archimonde with a bolt of his shamanistic lightning, an incredible feat in itself. With the two bases destroyed, Archimonde stormed through the night elf gate protecting the summit.
With Archimonde at the World Tree, Malfurion used the Horn to call on the ancient spirits in the area to attack the demon. In an instant, they detonated into a massive explosion that destroyed the Defiler, much of his demon army with him, and the World Tree. The price for this defeat was the night elves' immortality, but the Third War was over.
The Third War left Lordaeron destroyed, the Night Elves mortal, and the near-extinction of the High Elves. No large group of humans would ever regain control of Lordaeron, though the Argent Dawn and Scarlet Crusade have been fighting the Scourge in order to retake their land. Prince Kael'thas Sunstrider of the High Elves returned to his ruined city and renamed his people the Blood Elves in memory of their fallen comrades. The Blood Elves would eventually retake much of Quel'Thalas, and in time would even reactivate the power of the Sunwell. After the war, Sylvanas would break free of the Lich King's control, and form the Forsaken. One human commander, Garithos, has gathered the remnants of the Alliance still remaining in Lordaeron and retaken Dalaran and the Alterac mountains for the Alliance. After that Garithos formed an uneasy truce with Sylvanas Windrunner and together they assaulted the capital city of Lordaeron. The Dreadlord Balnazzar, who controlled the city, fought against their attempts to retake the city. After intense fighting and careful planning, the Alliance and the Forsaken managed to defeat his forces and regain control of the city. After the battle, Sylvanas ordered her second-in-command Varimathras to execute his brother Balnazzar and then to kill Garithos which forced the Alliance forces to retreat. The Forsaken then took over the city of Lordaeron, making their home in its sewers, calling it the Undercity.
The political scene of the current Azeroth was greatly influenced by the Third War. Under Warchief Thrall, the orcs, tauren, and Darkspear trolls were brought together on Kalimdor to become the new Horde, to which the Forsaken and the blood elves would later join. A peace agreement was made between the Horde and Alliance after the war, and Thrall has made it his job to ensure that nothing happens to break this already stressed pact. However, after the Wrathgate incident, Varian Wrynn declared war upon the Horde, to Thrall's lament.
Without Lordaeron, the rebuilt city of Stormwind would become the base of the Alliance, with the Ironforge dwarves becoming their closest geographical allies. The gnomes of Gnomeregan had been unable to help in the Third war as they were dealing with their own issues closer to home — namely a trogg invasion of their city. Because an invasion of troggs paled in comparison to the might of the Scourge, the gnomes decided they would fight their war alone. In the end they were forced to flee Gnomeregan and, with their king Gelbin Mekkatorque, settled in Ironforge with the dwarves. The night elves, dealing with the loss of their immortality, and having been exposed to the world outside their forests once more, decided they would have to make alliances with the wider world. Because of the animosity with the orcs for killing Cenarius, the night elves in the end joined the Alliance. As the new leader of the Alliance the King of Stormwind Varian Wrynn sent troops from Stormwind to repel the scourge invasion retaking Southshore and Hillsbrad an for the Alliance, repeling the Scourge on the Alteract mountains constructing a Final boarder point (Chillwind Camp) for the Alliance before The entrance of the Plague lands and siezing the Forsaken take over of Lordaeron. The human refuges of Lordaeron were given shelter in Stormwind making it their new home. a few months later, King Varian was kidnapped by the Defias Brotherhood after trying to make a peace agreement with the Horde making him absent for 3 years. In his absence, his 10 year old son Anduin was crowned as king.
The losses were not in vain, however, as the Burning Legion was heavily damaged. Many senior commanders were killed in the war, including a large number of dreadlords and countless others. As Aegwynn put it in the Cycle of Hatred to Theramore Guard colonel Lorena, they "did more damage to the entirety of demonkind than has been done in thousands of years."