Fenric

Fenric was an immensely powerful sentient force encountered by the Doctor. Fenric was at least as old as the Universe itself, an intelligence of pure evil.

Early history
Fenric was one of two forces, one good, one evil, which were present at the birth of the Universe. Fenric, the evil force, began influencing the new universe almost immediately. The fate of the good force remains unknown.

The Doctor's first known encounter with Fenric was in Constantinople in the middle of the 3rd century AD, when the Time Lord challenged the entity to a game of chess, knowing that the metaphorical struggle would weaken Fenric. The ploy worked and the Doctor was able to trap Fenric in a flask and banish him to the Shadow Dimensions. Yet Fenric's power was such that his influence still continued to pervade space and time, and he began setting into motion the events that would ensure his escape.

Fenric summoned the Haemovore, a vampiric human mutant from half a million years in Earth's future. When Vikings stole the flask containing Fenric sometime after the 10th century, the Haemovore followed them and the flask to Northrumbria and Maiden's Point. Fearing its evil influence, the Vikings buried the flask beneath St. Jude's Church, leaving a runic inscription warning that the flask was cursed. The Haemovore remained beneath the waters of Maiden's Bay, gradually building an army of vampires and waiting for its master's release.

Meanwhile Fenric continued to manipulate the timelines in ways that would ensure his release. He assisted Lady Peinforte in her use of black magic to travel to 1988 to retrieve the Nemesis statue, perhaps knowing that doing so would better ensure the Doctor's survival of this latest encounter with the Cybermen. He also created the time storm which engulfed Ace and transported her to Iceworld, where she met the Doctor and joined him on his travels.

Escape and defeat
In 1943 Fenric finally escaped, killing and occupying first the body of Doctor Judson and then that of Soviet Captain Sorin. He then summoned the Haemovore, whose vampire army arose from the waters and attacked and killed many of the villagers and base personnel at Maiden's Point. The Doctor attempted to trap Fenric once again using a chess game, but by possessing Sorin Fenric learned the solution to the chess puzzle. Fenric was finally defeated when the Haemovore turned on him, destroying Sorin's body and leaving Fenric without a corporeal host. Fenric's spirit dissipated, although it is likely it's evil influence still persists.

The Expanded Universe
Fenric received his name from the Vikings, who likened him to the Fenris Wolf. The Virgin New Adventure novels establish that Fenric was in fact Hastur the Unspeakable, one of the Great Old Ones.