Centro

Centro was a being who resembled a metal skull with camera-like eyes.

Centro created spacewarps which threatened the universe. The Doctor was forced to trap him somewhere where he could not escape. (PROSE: The Infinity Doctors)

Whilst pondering who could be responsible for the destruction of Gallifrey, Marnal mentioned Centro. (PROSE: The Gallifrey Chronicles)

Behind the scenes

 * The fanzine archive Time, Unincorporated: Volume One reveals that Centro was the antagonist of the version of The Infinity Doctors meant to work in tandem with Kate Orman and Jon Blum's Mentor. Centro would have played a very similar role to that of Savar in the finished novel.