An elf (from the Old English ælf, elf, ylve, etc.) is a type of supernatural being in Germanic mythology and folklore. Early elves, whose description depends almost entirely on Norse mythology texts, were a race of beings with magical skills, ambivalent towards humans and capable of either helping or hindering them. But Christianized societies were viewing elves in increasingly sinister light. In Anglo-Saxon England as early as the 10th century, Old English medical books attest to elves afflicting humans and livestock by “elf-shots”. The German elf or alp was seen as an “addler" of people in medical books, but already in the High Middle Ages there were prayers warding against it as the agent causing nightmares, and eventually for the alp its identity as nightmare spirits became predominant.