Gwyll

Gwyllion are a fictional sapient species originating in Welsh folklore.

Description
Gwyllion appear as ugly old women wearing tattered clothes and a four-pointed hat, most often seen carrying a pot. In mountain passes, Gwyllion like to confuse travelers with randomly shouting "Wb!" from various directions, causing them to try to follow them and eventually get lost. Gwyllion occasionally come into a human house for food and shelter, and will leave harmlessly if its demands are met. If a Gwyll proves hostile, she can be repelled by the sight of an iron knife.

Behind the scenes
Gwyllion's true nature is heatedly debated. The word 'gwyll' is etymologically related to 'ghost', leading some to assume Gwyllion are a type of undead. However, the word is sometimes translated as "mountain fey", and the idea of Gwyllion being fairies square better with their behavior, such as their hatred of iron.