Prelude
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'zhuls 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 prophets 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.
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
(Main article: Culling of Stratholme)
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.
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.
After Stratholme was purged, some former paladins, now called Deathlords, committed the same heinous act in many other cities across Lordaeron.
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. The two armies then established a new camp and fought together against the undead, famished and vicious packs of wolves, strange beings that seemed to be part wolverine and part human, as well as ice trolls.
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 fought and killed his former mentor Uther for the urn he was carrying. The urn carried the ashes of his father, which Arthas 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
(Main article: Scourge invasion 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, rather than give 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. Upon eradicating the remnants of the Blackrock clan in Alterac Mountains, 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 take control of the second Well of Eternity under the World Tree Nordrassil.