You are hereEllylldan
A Welsh spirit similar to the English Will o' the Wisp, it appears as a light and misleads travellers from their path.
Along with black dogs, tales of fairy lights are common throughout Britain, with a different name given to a similar phenomena. In general they are seen as malevolent, guiding lone travellers into treacherous bogs.
The belief may arise from the natural production and combustion of methane in boggy areas.