Scourging of Lordaeron

The Scourging of Lordaeron was the beginning act of the Third War and carried out by the Undead Scourge under the command of the Lich King, who in turn served the Burning Legion.

After swearing allegiance to the Lich King, the necromancer Kel'Thuzad founded the Cult of the Damned which spread a plague throughout Lordaeron that turned those infected by it into the undead. In response, Prince Arthas, the son of Terenas Menethil II, was sent to investigate the plague along with Jaina Proudmoore. They discovered that the infection spread through plagued grain and succeeded in killing Kel'Thuzad, but were unable to stop the plague from spreading. After discovering Stratholme had already been infected, Arthas ordered the purging of the city and suspended his mentor Uther the Lightbringer, along with the Order of the Silver Hand, for refusing to carry out the order and did so himself. Arthas traveled to Northrend to confront the dreadlord Mal'Ganis who controlled the Scourge. With the power of the runeblade Frostmourne, Arthas was able to defeat Mal'ganis but lost his soul in the process. He returned to Lordaeron and murdered his father King Terenas, then oversaw the undead overrun Capital City and the fall of the kingdom.

Arthas, now a death knight, led the Scourge in the destruction of Quel'Thalas and Dalaran to prepare for the Burning Legion's invasion. Following the defeat of the Legion at the Battle of Mount Hyjal, the Scourge became free from its control and Arthas returned to Lordaeron to declare himself king and scour the last remnants of humanity. However, the Lich King's powers began to weaken and so did his control over the undead. Arthas returned to Northrend and shattered the Frozen Throne, put on the Helm of Domination, and merged with the Lich King.

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'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.

After preparing for many long months, Kel'Thuzad and his Cult of the Damned finally struck the first blow by releasing the plague of undeath upon 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 and told him to save his people, he must travel west, Terenas told him that he would remain here and 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.

After Stratholme was purged, some former paladins, now called Deathlords, committed the same heinous act in many other cities across Lordaeron. After years of constant war and suffering, some members of the clergy also lost their sense of the Holy Light.

In the days since Prince Arthas took the fleet to Northrend, the Silver Hand continued to defend Lordaeron villages. They were stretched thin fighting the spread of the plague. However, the Scourge continued to grow, taking villages and its turned inhabitants, with a small help from Baron Rivendare and his corrupted food supplies. When a village that Thomas Thomson was stationed in was turned, Thomas understood that Rivendare was a traitor and reported it to the rest of the Silver Hand paladins.

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 where he lost the rest of his humanity, becoming a death knight. When he returned to the base camp, he slaughtered his men, raising them as mindless Scourge and death knights.

At the same time, the undead mysteriously vanished from the Eastweald. Most of the citizens thought that this meant Arthas had succeeded in destroying the Scourge in Northrend. In truth, the Lich King had ordered his forces to pull back in order to make the population believe that Arthas had defeated the undead and trick them into welcoming him back with open arms. Arthas returned to Lordaeron to get his just rewards. With the presence of Falric and Marwyn, the three enter Capital City clocked. The people of Lordaeron doused them with rose paddles as they entered the throne room. Terenas Menethil II greeted his son with open arms Arthas and, without a moment of consideration, slew his father. Soon, Capital City was engulfed by the undead and it fell soon after. During this time, Arthas ordered Thassarian to kill his mother as an act of loyalty, an order that the death knight carried out by decapitating his mother. Following the murder of his father, Arthas journeyed to the grave of his fallen steed, Invincible, and raised him from the dead.

When the Scourge attacked Tirisfal, the Agamand family fortified their mills. However, Devlin betrayed his people and made a deal with the Scourge. After Capital City fell, the Scourge army swept across Lordaeron. Neighboring human and dwarven nations converged on Lordaeron to vanquish the Scourge. Even the elven kingdom of Quel'Thalas, no longer an Alliance member, sent their priests to help defeat the undead. The Scourge was numerous and defeated any Alliance resistance. Ultimately, the Scourge took Lordaeron, but small pockets of Alliance resistance remained. In the meantime, Jaina Proudmoore was gathering Lordaeronians to sail for Kalimdor. The Knights of the Silver Hand had to stay behind to defend those that couldn't get on the Kul Tiran ships, as well as continue fighting against the Scourge. When Alexandros Mograine returned from Ironforge with the mighty Ashbringer in hands, the Highlord was reported to be paring down the Scourge's numbers, and did so for several months.

Despite this, in the following days, Arthas, 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 in Andorhal 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 on the Isle of Quel'Danas, past Silvermoon City, capital of the high elves. To revive Kel'Thuzad, the Scourge would have to burn Quel'Thalas.

As Andorhal fell, its desperate defenders, seeing no hope of rescue, had set the city ablaze so that the Scourge couldn't raise more warriors.

Following the death of Antonidas and the destruction of Dalaran, the newly summoned Archimonde ordered his forces to gather in the heart of Lordaeron and crush any vestiges of the Alliance so that no one would follow them on Kalimdor.

Aftermath
Even with Arthas departing for Quel'Thalas the damage done to Lordaeron and the Scourge forces left behind ensured the land stayed with the dead. The devastating attack saw refugees fleeing west with Jaina Proudmoore to Kalimdor and heading south to the safety of Stormwind and Ironforge. However, life still remained within the continent as resistance movements against the Scourge were being created in the form of the Scarlet Crusade, Argent Dawn, and the Alliance resistance. At the same time survivors Knights of the Silver Hand Halahk the Lifebringer, Magroth the Defender, and Dagren the Orcslayer began to serve as a protect force for those that remained within the heart of Capital City.

Following the Battle of Mount Hyjal King Arthas would direct the Scourge (now free from Legion influence) to scour the last remnants of humanity from the Lordaeron. As such the Knights of the Silver Hand and their forces started to direct the human villagers to escape into the safety of the mountains. However, Arthas anticipating this had stationed his forces to block the mountain passes. Over a relatively small amount of time the Scourge under Arthas, Kel'thuzad, and Sylvanas Windrunner would destroy the Circle of Holy Light, the Defenders of Justice, and the Crippling Force. However, Dagren would survive and rally others to him and join Admiral Proudmoore and journey to Kalimdor.

At the same time a civil war within the undead (caused by the Lich King's weakening powers) would see the Alliance resistance enslaved by Detheroc and Balnazzar, and then entering into a partnership with the fledgling Forsaken after being freed. The Alliance resistance would ultimately be destroyed following the "death" of Balnazzar, leaving the Scarlet Crusade as the main resistance movement against the undead, both Scourge, and Forsaken, in the continent (the Argent Dawn lower in number and just focused on Scourge).

Arthas returned to Northrend to reach the Lich King at the Frozen Throne as both were continuing to lose their powers. He raced against Illidan Stormrage and Kael'thas Sunstrider, who wanted to destroy the Lich King. With the help of the crypt lord leader Anub'arak, Arthas was able to arrive at Icecrown from underground. After defeating both Illidan and Kael'thas, along with their forces, Arthas ascended to the top of Icecrown Citadel where the Frozen Throne was located and shattered the Lich King's icy prison with Frostmourne. Arthas then picked up Helm of Domination, placed it upon his head, and merged with the Lich King.