Annwfn

He is created in the lowest stage of life, in Annwfn, the shadowy abyss at the base of Abred.

Another name applied to the Fairies is Plant Annwfn, or Plant Annwn. 

A poem speaks of the magic cauldron of Pen Annwfn, i.e.

Where the Tylwyth Têg threw me . . . if I had not come in time to rescue thee from the clutches of Plant Annwfn.

Annwn, or Annwfn is defined in Canon Silvan Evans’s Dictionary as an abyss, Hades, etc.