Tannis
Tannis was a Time Lord who became 'Supreme Commander of the Defence Forces of the Canisian Suns', with the rank of General. His plans to infect a planet with a deadly plague were thwarted by a visiting stranger known to the inhabitants as 'Manaster', most likely the Minister of Chance, following which he agreed to the Treaty of Karselai and signed away any rights to alien planets. Despite this, the Canisian military, unaware of his true identity, conquered the Santine Republic, brutally obliterating the forces of Admiral Mettna, murdering the President and confining the leading members of the Republic to a prison camp. He also took control of the home planet of Alpha Canis One by holding hostage the infant son of Canisian Premier Bedloe. (WC: Death Comes to Time)
Tannis sent the assassin Nessican to Earth to kill Valentine and Antenor - Time Lords known as 'The Saints' - and plotted to exterminate all remaining members of the Fraction. He orchestrated the Minister into breaking the rules of non-interferance, requiring the Doctor to confront him, while Tannis himself killed Antimony and travelled to Mount Plutarch to murder Casmus. He then invaded Earth but was challanged by space shuttles commanded by Brigadier Lethbridge-Stewart, while his weapons were disabled by a computer virus engineered by the Doctor from Nessican's transmitter. In a final confrontation on Salisbury Plain, the Doctor appeared to kill both Tannis and himself. (WC: Death Comes to Time)
Larna mentioned Tannis to Ulysses, Saldaamir, Penelope Gate, and Marnal in a list of entities that would threaten Gallifrey in their future (her relative past). She said it was very important that Gallifrey should survive Tannis's attack to ensure that the world would be destroyed at its proper time. (PROSE: The Gallifrey Chronicles)
