Varnax

Varnax was an individual who 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)

Behind the scenes
The references to Varnax are references to the unmade movie in the 1990s which was to feature Varnax's attack on Gallifrey.