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" and "night-wanderers (human or supernatural) up to no good, outlaws of the wild."
It is a comparatively recent word coined inadvertently in the seventeenth century by the Welsh lexicographer Dr John Davies of Mallwyd.
[1] According to folklorist Wirt Sikes the gwyllion are female fairies of frightful aspect who haunt lonely roads in the Welsh mountains and lead travellers astray.
While it was desirable to exorcise them when in the open air it was not prudent to display an inhospitable attitude towards any member of the fairy realm.
Sikes speculates that if these ideas are traced back to their source then they may have some connection with the sword Excalibur and the role it plays in the primeval world.