Varimathras is the co-ruler of The Forsaken.
He is a Dreadlord, a race of demons from the Twisting Nether used as commanders for the invasions of the Burning Legion.
With the fall of the Legion at the Battle of Mt. Hyjal, he and several of his brothers were abandoned in the Plaguelands at the Ruins of Lordaeron. They had no news of the defeat of the Legion, and continued to rule over the undead stationed there while waiting for further instructions from Archimonde the Defiler.
Their fortifications were assaulted during the Undead Civil War by the forces of the Forsaken and their leader Sylvanas Windrunner, and he was captured by the Dark Lady. Out of cowardice, he swore fealty to her in order to preserve his own life and assisted her in the destruction of the Dreadlords Balnazzar and Detheroc. He personally killed his brother Balnazzar, leader of the Plaguelands' Legion forces. Since then he has assisted Sylvannas in her rise to power as ruler of the Forsaken, and co rules with her from the Undercity. He runs the day to day affairs of the undead faction, monitoring the progress of the Forsaken's war against the Scourge and the Scarlet Crusade.