In one version of reality, Tannis was a renegade Time Lord who abused his powers to take control of the Canisians and led them in a war campaign which climaxed with their invasion of Earth. The Seventh Doctor investigated Tannis and appeared to give his life to defeat him. (WC: Death Comes to Time)
A Gallifreyan prophecy foretold that Tannis would pose a threat to Gallifrey in its future. 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)