The concept of "Annwn" (Annwyn) is not made up especially for the film or the book on which it was based. "Annwn" is an underworld or other world found in Welsh legend, a land of the dead. It is said to lay far in the west and could be accessed by the living through a door located at the mouth of the Severn once a year. Surviving from pre-Christian Celtic mythology, it's neither Heaven nor Hell in the Christian sense, humans can enter spiritually or corporeally. This is the first film about Annwn.