The War King

The War King was an incarnation of the Master sworn to lead the Time Lords during the Second War in Heaven.

History
During the events of the Second War in Heaven the Master was captured and interrogated about his world by the agents of the Enemy. The Master managed to escape, but the torture deeply affected the Master, it made realise how he cared for his people and the Master concluded that he would help the Time Lords in any way he could. (FP: The Book of the War)