"Evil, evil since the dawn of time" is how the Doctor has described Fenric. And its origins: "The dawn of time. The beginning, all beginnings. Two forces, only good and evil. Then chaos - time is born, matter, space, the universe cries out like a newborn. The forces shatter as the universe explodes outwards. Only echoes remain. But somehow, somehow, the evil force survives. An intelligence, pure evil." This being has no name, but Commander Millington - with his obsessive interest in Norse mythology - calls it Fenric.
Fenric is a calculating and sadistic entity which takes great pleasure in killing. It requires a host body in which to dwell, and it retains the memories of the host. Fenric can vanish from one place and reappear in another, control the weather (it seems to have a fondness for thunderstorms), and switch host bodies if needed, though this process apparently requires that the two host bodies be either in contact or very close to it. Upon initially being taken over by Fenric, the host body's eyes glow a bright green, but this fades over time to a dull yellow. The strain of being taken over by Fenric is apparently very hard on the host body, particularly if Fenric must exert itself mentally, necessitating a periodic switch of hosts.
In approximately the third century AD, the Doctor encountered Fenric in a desert on Earth. He carved chess pieces from bones he found there, set up a board in play and posed a puzzle to Fenric: figure out one move that will win the game. Fenric was unable to solve it, and the Doctor trapped him in the shadow dimensions. Fenric was contained within a flask, where he remained until 1943.
Certain human beings, possibly those who have become entangled in Fenric's plans, are "touched" by the entity, and become "wolves of Fenric". The Vikings who brought Fenric's flask from Europe to Maiden's Point and all of their descendants were wolves of Fenric, as were Judson, Millington, Sorin, the Ancient One...and Ace, through her mother Audrey and grandmother Kathleen Dudman. This taint enabled Fenric to engineer Ace's future; it caused the time storm that took her to Iceworld and put her in the Doctor's path.
Fenric is finally released from the shadow dimensions when a runic inscription, found in a crypt under the church where the flask was buried, is translated. It takes over Judson's body and summons the Ancient One, the oldest of the haemovores, to do its bidding. The church is built near a reservoir of natural toxin which has been stockpiled for use in the war, and Fenric plans to have the Ancient One release a large quantity of this poison into the water and contaminate the world, thus creating its own future. Meanwhile, the Doctor poses the game to Fenric again. Unable to resist, Fenric weakens Judson's body and takes over Sorin, but it is still unable to divine the solution until an inspired Ace presents it to what she thinks is still Sorin - the black and white pawns don't fight one another, they join forces. Having solved the puzzle, Fenric believes it has triumphed over the Doctor, but it does not realize until too late that the Ancient One does not wish to destroy the Earth. The haemovore drives Fenric back into a sealed room and releases a canister of the poisonous gas, killing the host body.
