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)

Indeed, in one reality the Seventh Doctor investigated Tannis whilst waging war upon the Canisians, and eventually appeared to give his own life while killing him. (WC: Death Comes to Time)