Fenric

An ancient evil, existing since the very dawn of time and one of the Doctor's oldest enemies. Tricked by the Doctor during a game of chess during the third century, Fenric's life essence had been trapped in a casket and abandoned for centuries.

Brought to England by the Vikings, Fenric manipulated the genetics of the Vikings, creating a long line of descendants all of whom would be loyal servants.

In 1943, Fenric was freed, having placed one of the descendants, Ace, aboard the TARDIS.

However, trapped inside a human body, Fenric was destroyed by the ultimate descendant, the Ancient Haemovore from thousands of years in the future.