Tannis

Tannis was a threat to Gallifrey in its future, as foretold by a prophecy. Larna mentioned him to Ulysses, Saldaamir, Penelope Gate, and Marnal among other threatening entities; she said that Gallifrey must survive Tannis' attack to ensure that the world would be destroyed at its proper time. One Time Lord would be central to defeating Tannis' attack and all of the other threats. (PROSE: The Gallifrey Chronicles)

Behind the scenes
Tannis appeared in NOTVALID: Death Comes to Time, where he was voiced by John Sessions. The Seventh Doctor investigated Tannis while he was waging a war upon the Canisians; during the investigation, he murdered Antimony. After Tannis revealed himself to be a Time Lord, the Doctor appeared to give his own life while killing him. (NOTVALID: Death Comes to Time)