The Fiend is the Dark made Flesh,also called Old Nick and The Devil. He has control over time, and he can change his shape at will. He claims to be Alice Deane's father, this is later proven true by Tom Ward's Mam. There are few ways to keep the Fiend from approching you, one way is to bear him a child, (witches do this often) and the other is to have a blood jar, which contain's one of the Fiend's child's blood, and the wearer's blood.