Brother Thomas was a Franciscan friar.
Aliens stranded in 1346 England, after discovering from a temporal scan of the upcoming Black Death, sent the mind of one of them back to 1278 to encourage Roger Bacon to develop a cure. Thomas was the person in whom the alien took mental residence. It made him kill anyone who would interfere in Bacon's research, like Brother Godwin and Brother Hubert. He tried to kill Nyssa as well, but she struck him in the head with a metal knife, which drove out the alien.
Thomas confessed to his crimes and was expelled from the order. He lived long enough to find the aliens in 1346 and try to warn them, but they thought he was mad and ignored him. (PROSE: Asylum)