Watcher Master

The "Watcher Master" was a construct of the Master's TARDIS, which the Master likened to a Watcher. It had the appearance of his decayed thirteenth self. Partially possessing and manipulating the Watcher, the Master was able to execute a plan to steal one of the Fifth Doctor's future regenerations. Existing only as darkness maintained by his will, the Master merged with the Watcher to gain a body, later stealing a little bit from each of his past selves and once again regaining the body of Tremas. (PROSE: The Velvet Dark)