Tannis

Tannis was a Time Lord who became a General in the Canisian fleet.

Biography
Tannis was known to Casmus as "the slow agent". He was the evil to balance the good of the Fraction. He ordered Nessican to kill Valentine and Antenor and personally killed Antimony and probably Casmus.

His intention was to kill the other Time Lords so he alone would have the power. He manipulated the Minister of Chance into using his power to destroy Canisian ships on Santiny so the Seventh Doctor would be forced to remove the Minister's power. Tannis then invaded Earth and tracked Ace to Salisbury Plain. The Doctor arrived and used the power to apparently kill Tannis and himself, leaving Ace to inherit the mantle of the Time Lords. (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. (EDA: The Gallifrey Chronicles)