Undercity was the capital city of the forsaken. It was built underneath the ruins of Lordaeron Castle. It originally served as a crypt to the royal family of Lordaeron. When Arthas Menethil became a death knight in the service of the Lich King, he killed his father, King Terenas Menethil and claimed the Undercity for himself. It was from there that he launched numerous campaigns against the forces of Azeroth. After the defeat of Archimonde at Mount Hyjal, Arthas was summoned back to Northrend to defend the Lich King from Illidan Stormrage and his army. During this time, a legion of undead forces, led by Sylvanas Windrunner, broke free of the Lich King's command and fought against their former master. After the Lich King was defeated, Arthas took up the mantle, becoming the new Lich King and thereby abandoning the Undercity. Sylvanas Windrunner led her army against the remaining contengent scourge forces. Sylvanas was successful and claimed Undercity as her seat of power. Sylvanas then realized that despite her victory, she needed allies. Knowing that the Alliance would never accept them, she turned to the Horde. She convinced the tauren that all the forsaken wanted was redemption. Realizing that the Horde needed allies, Warchief Thrall reluctantly agreed to an alliance with the forsaken.