As The Lich King was losing his power, he also lost control over much of the Scourge. Varimathras and his brothers, Detheroc and Balnazzar took control over these undead and usurped control of Lordaeron from King Arthas Menethil.
After Sylvanas Windrunner failed an assassination attempt on Arthas, Varimathras greeted her personally and offered her a chance to join them. She refused and instead destroyed Varimathras' base. After his defeat, Varimathras pledged loyalty to Sylvanas and helped her defeat his brothers. He supposedly proved his loyalty by personally killing Balnazzar.
Sylvanas then took control of Lordaeron and became the leader of the newly-formed Forsaken with Varimathras as her second. The Dreadlord then led his own branch of the Forsaken but he betrayed her and usurped control of the Undercity. After he was defeated, the remainder of his forces were either destroyed or absorbed into the Forsaken.