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After Archimonde's defeat at the Battle of Mount Hyjal, there were several demonic forces still scattered across Azeroth. Some remained unaware of the defeat of their masters. Many of the dreadlords in Lordaeron in particular did not know of what had unfolded across the sea, despite a span of many months.
Arthas returned to Lordaeron to claim his throne, but found that the Scourge was under the dominion of the dreadlord jailors. When Archimonde had been summoned into Azeroth, Tichondrius had asked that the Scourge be given to him, and thus it was the dreadlords who were the new custodians of the undead.
However, without Archimonde, the dreadlords were exposed. Arthas drove them away, for the time being. Unfortunately for him, the Lich King was growing weaker. Control over the Scourge was lost, for the most part.
The Dreadlord Insurgents were the result of the Lich King's will weakening, coupled with manipulative power. Most of the undead who were freed were in turn enslaved by the dreadlord custodians who were controlling the Scourge in Lordaeron. Balnazzar, Detheroc and Varimathras were the leaders of this Insurgency.
The splintering of the Scourge was to free Sylvanas Windrunner, with whom the dreadlords made a pact. With the assistance of Sylvanas, the Dreadlord Insurgents prepared a trap for Arthas. Arthas was ambushed in Capital City and was separated from his majordomo, Kel'thuzad. Despite this, he survived the onslaught and Sylvanas' assassination attempts, and sailed to Northrend.
This left a power vacuum in Lordaeron. There were three undead factions at large. One was the Scourge under Kel'thuzad's control. Another was the precursor to the Forsaken; Sylvanas and her banshees. Finally there were the Dreadlord Insurgents, retaining control over large unholy armies, including the mind controlled soldiers of the Lordaeron Resistance under Garithos.
Kel'thuzad chose not to interfere and was not seen for the most part of the Scourge Civil War. Sylvanas, on the other hand, was offered a place in the new order that was being forged by the Insurgents. She refused, and Varimathras launched an assault upon her base. Sylvanas improvised, and possessed local creatures using banshees to turn them against the Insurgents. In this manner she swayed Mug'thol, Blackthorn, the Murloc Puddle Lord, Snarlmane and possibly Zul'rogg, . This gave her several factions of beasts to throw against Varimathras. Eventually, she was successful, and Varimathras joined her in exchange for his life.
The Dreadlord Insurgents still controlled much of Lordaeron, however. Detheroc's pet Garithos was protecting his master within a fortified city. With her banshees at hand, Sylvanas possessed the human gate guards and turned them on one another, allowing her army into the city by cover of night. While the soldiers of the Lordaeron Resistance slept, Sylvanas set about killing her enemies and taking the city. By the time they were alerted to her presence, it was too late, and the city was soon taken and Detheroc torn to bloody shreds.
Ultimately, it was in Capital City that the Dreadlord Insurgency met its end. Sylvanas forged an alliance with the Lordaeron Resistance, promising Garithos that he could have Capital City in exchange for help in killing Balnazzar. Garithos agreed, and despite tensions, Sylvanas and the Resistance stormed Balnazzar's fortifications.
Balnazzar proved a tenacious foe, throwing both demons and undead at his enemies, but he was surrounded and the city was destined to be taken. Sylvanas ordered Varimathras to kill Balnazzar, and it appeared that he did so. In truth, Balnazzar survived, going on to haunt Stratholme, where he killed Saidan Dathrohan and possessed his body. Sylvanas betrayed the Alliance and ordered Varimathras to kill the Grand Marshal, Garithos, which he gladly did. From that moment onward, the Forsaken were declared, and they became the dominant undead force in Lordaeron. As time passed, Kel'thuzad was to return to action and waged his war against the Forsaken, but by then the rebel undead were firmly entrenched.
The Dreadlord Insurgency lives on in the form of the Scarlet Crusade.This section concerns content exclusive to Wrath of the Lich King.
During the Battle for the Undercity the Dreadlord Insurgents returned in the form of an undead and demonic army under the command of Grand Apothecary Putress and Varimathras. The Dreadlord Insurgency, including the leaders, was destroyed during the battle.