From Wikipedia:
Redcap, also known as a powrie or dunter, is a type of malevolent murderous dwarf, goblin, elf or fairy found in Border Folklore. They are said to inhabit ruined castles found along the border between England and Scotland.
Redcaps are said to murder travelers who stray into their homes and
dye their hats with their victims' blood (from which they get their
name).
Redcaps must kill regularly, for if the blood staining their hats dries
out, they die. Redcaps are very fast in spite of the heavy iron pikes
they wield and the iron-shod boots they wear. Outrunning a redcap is
supposedly impossible.