Catavolcus

Catavolcus was a warlord who once fought the Doctor. (DWM: The Neutron Knights) He also attacked Gallifrey some time before its first destruction in the Second War in Heaven against The Enemy. (EDA: The Gallifrey Chronicles) The Time Lords knew of this long before the attack and knew it was a threat they were destined to survive. (PDA: The Infinity Doctors)