Vandekirian

Vanderkirian was an ancient Gallifreyan and a contemporary of Rassilon and Omega. He was Omega's assistant and was remembered as the one who betrayed him.

Biography
Historically accounts depicted that Omega's assistant, Vanderkirian, had sabotaged the mission on behalf of Rassilon. However, feeling guilty over his betrayal, he cut off his own hand as recompense. Omega did not accept it, and cut his other hand, placing it in his stellar manipulator, which was later to be known as the Hand of Omega. Upon which, the Eurydice was destroyed due to Vanderkirian's actions. (AUDIO: Omega)

In his own distorted memories, Omega recalled that Vandekirian was at least partly responsible for the sabotage. He had discovered that detonating the star would jeopardize the existence of a race called the Scintillans. When he realised what would happen, his guilt caused him to destroy his own hand, since it's activation required his palm print. However, Omega cut off his other hand to activate it. Although, the Doctor revealed that the history with the Scintillans was a corruption of his own memories, over the guilty of accidentally killing the race himself, and incorporated it to reconcile his reasons for Vanderkirian's betrayal. The Doctor hypothesized that Vandekirian had either sabotaged the mission on behalf of Rassilon, or he had just succumbed to madness. (AUDIO: Omega)