Varimathras is a Nathrezim demon who originally served the Burning Legion. He was forced into allying with the Forsaken, under the direct control of Sylvanas Windrunner, in exchange for sparing his life after he and his fellow Nathrezim were overcome by her undead forces. Though he swore allegiance to Sylvanas, Varimathras remained loyal to the Legion and eventually staged a coup for control of the Undercity with Grand Apothecary Putress.
During the Valiance Expedition and Kor'kron Vanguard's combined attack on the Lich King at Angrathar the Wrathgate in Northrend, Varimathras and Putress launch a surprise assault, which ends up supposedly killing Bolvar Fordragon and injures the Lich King, forcing him to retreat. Warchief Thrall and Sylvanas Windrunner lead a party of Horde warriors to the Undercity to take it back from Varimathras and Putress' forces. In the end, they are sucessfull, and Varimathras is banished back to the Twisting Nether.
He is still believed to be within the Twisting Nether, plotting a return to Azeroth.