For the Belgian band, see Gwyllion (band).
Gwyllion or gwyllon (plural noun from the singular Gwyll or (Yr) Wyll "twilight, gloaming") is a Welsh word with a wide range of possible meanings including "ghosts, spirits," "night-wanderers (human or supernatural) up to no good, outlaws of the wild" etc. Gwyllion is only one of a number of words with these or similar meanings in Welsh. It is a comparatively recent word coined inadvertently in the 17th century by the Welsh lexicographer Dr John Davies (Mallwyd). They may also be known as Mountain Fey, Fae, or fairies.
One account of a species of gwyllion refers to them as ugly female spirits that are usually described as wearing ash colored with an oblong four-pointed hat, and usually carrying a pot in one hand. They also are known by their disturbing laughter and their cries of "Wb!" . They are said to live in mountainous areas and love to mislead travelers or scare them. However, if one of these gwyllion in disguise enters your home and is treated right, it is believed they will do no harm. These gwyllion are supposedly repelled by metal knives, and a flash of a knife is thought to usually be enough to send them away.