Name: Fenris the Hellbringer
First Appearance: Doctor Who Magazine #47 (Dec. 1980): "Star Death, Part 1"
Number of Appearances: 2
Employer: Order of the Black Sun
As far as I have been able to ascertain, this character is an alien, although he appears entirely human. His name is notable, however, as he is named after the ferocious wolf from Norse/Teutonic mythology.
Fenris was a time traveler and a mercenary from Gallifrey's relative future at the time of Rassilon.
Fenris was an agent of the Order of the Black Sun (the great enemies of the Timelords), but it does not appear as though he actually belonged to the Order himself. He was hired by them to do their bidding.
As an agent for the Black Sun, Fenris sabotaged Omega's defense shields. This act sucked him into a black hole, dispersing his atoms across the Zone of No Return. It seemed certain he was gone forever. Yet twenty years later, he was retrieved and reassembled by Wardog for interrogation. While he was being interrogated, Fenris was killed when Black Sun attacked Gallifrey, the homeworld of the Timelords.
Suggested links for further research about this character and the character's religious affiliation: