Varnax

Varnax was an individual who attacked Gallifrey some time before its first destruction in the War against the Enemy. (PROSE: The Gallifrey Chronicles) In one universe, the Time Lords knew Varnax was a threat they were destined to survive. (PROSE: 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.