In the wake of Illidan's failed attempt to melt the icy continent of Northrend, the powerful energies possessed by the Lich King inside his Frozen Throne slowly began to decay. Inexorably this resulted in a partial loss of control of the more distant Scourge forces. The result was that many undead under the Lich King's mental domination had their conscious will restored. Their spirits and memories were somehow returned to their undead bodies. Even the Lich King’s champion, Arthas, began to weaken as the Lich King's power waned. Arthas the Death Knight and his lieutenant, Kel'Thuzad the Lich were suddenly thrust into an undead civil war. Still fanatically loyal to the Lich King, Arthas heeded the call of his master and returned to Northrend as Illidan launched his second attempt to destroy the Lich King directly. Arthas left Kel'thuzad in command of his forces when he departed.
With Arthas's departure from Lordaeron, the three Dreadlords Balnazzar, Varimathras and Detheroc attempted to regain control over the undead forces in Lordaeron using their formidable mental powers. The former High Elf Sylvanas Windrunner rallied many of the newly freed undead to counter the Dreadlords' efforts. Sylvanas was amazingly successful and not only crushed the Dreadlords' forces, but also utterly destroyed the last major contingent of human forces in Lordaeron. She forced Varimathras into her service in exchange for sparing his life and took control of the ruined capital of Lordaeron as her own. Within the sewer system of the ruined city Arthas had constructed his throne room; but with Arthas now gone, Sylvanas claimed this Undercity as her capital and set out to expand her ranks by freeing even more undead.
Dubbing her band of free-willed undead the Forsaken, Sylvanas worked to ensure that the damned such as herself would have a home free from threats by the living. Whether she plans a cure for undeath or to bring suffering to all the world is a question whose answer only Sylvanas knows. What is known is that the Horde sympathizes with the Forsaken. The Orcs, having been enthralled to demon masters for a generation, felt an obligation to help similarly liberated people. While the Horde does not fully trust the Forsaken and vice versa, it is a start.