Fenric was a being described by the Seventh Doctor as "evil from the dawn of time", a malevolent force that survived the clash of energies present at the birth of the universe. In an untelevised adventure, the Doctor had encountered Fenric and defeated him by challenging him to solve a chess puzzle. When Fenric proved unable to solve it, the Doctor then trapped the being in a flask where he remained for over a thousand years.
However, Fenric was still able to manipulate human minds and events through time and space. He set up pawns, bloodlines of families that were under his control and he could use, "The Curse of Fenric" stretching down through generations. These people were known as the "Wolves of Fenric", and their true purpose was unknown even to them. He also had the power to summon Haemovores, vampires which were to be the evolutionary destiny of mankind in a possible far future. The haemovores were strong enough to be able to weld metal with their bare hands, and were also immune to bullets. They could be countered, however, with a psychic barrier caused by faith.
Eventually, the flask was brought to a military base in Northumberland in 1942, where several Wolves, including the Doctor's companion Ace, were manipulated into freeing Fenric from his flask. He also summoned the Ancient One, the last of the Haemovores from the future, in an attempt to poison the world with a deadly chemical toxin. Fenric then revealed that he had manipulated the Seventh Doctor's life upon several occasions as part of his game, including creating the time storm that originally took Ace to Iceworld and influencing Lady Peinforte in her attempt to gain the power of the Nemesis statue. Eventually, the Doctor convinced the Ancient One to turn on Fenric; the Ancient One then destroyed Fenric and himself with the same toxin.
The Virgin New Adventures novel All-Consuming Fire by Andy Lane also equates Fenric with the Cthulhu Mythos entity Hastur the Unspeakable.
Fenric and his Haemovores return in the 2012 Big Finish Productions audio story, Gods and Monsters.