Catavolcus

Catavolcus was a warlord. He once fought the Doctor, (DWM: The Neutron Knights) he also attacked Gallifrey sometime 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)