Skellis was a Time Lord who fought in the war with the Great Vampires.
She grew up with Gith, the both of them training at the Academy together.
On the planet Lerna, Skellis and Gith were attacked by Macra. One of the Macra decapitated Gith, and Skellis murdered it in revenge. Nearby was a weakened Great One, which Skellis was unable to kill as her weapon had spent its ammunition. Skellis took the claw of the Macra and stabbed it through the heart, and watched with surprise as it turned to dust before her eyes.
In a male body Skellis brought this new-found knowledge to his superiors, and was promoted to General. Skellis was brought on board the Golden Sword project on Phonida, and helped with the discovery that the old Time Lord tactic of dispersing Vampires across time and space actually led to their spreading in the first place.
The Skellis Gun was named after Skellis, and was instrumental in the defeat of the Vampires. Rassilon and Skellis became good friends, and visited him whenever he returned home to Gallifrey. (PROSE: The Multi-Faceted War)