He forshadows the Grail Quest in "The Spoils of Annwfn".    From Wordnik.com.  [Camelot Frequently Asked Questions List Version 1.1 by Chris Thornborrow] Reference

The Four Branches of the Mabinogi: The First Branch is set in Dyfed, an area connected to the magical underworld known as Annwfn.    From Wordnik.com.  [Archive 2007-01-01] Reference

Medieval people knew that world well; in medieval Wales, for instance, it was named clearly: Annwfn, the Inside Place, the In-world.    From Wordnik.com.  [Writer Unboxed » Blog Archive » NATURAL MAGIC: inspiration from the animal world] Reference

Pwyll Pendevic Dyfed, the hero, undergoes magical trials before emerging as the Head of Annwfn and consort of the Great Queen, before she gives birth to the hero Pryderi.    From Wordnik.com.  [Archive 2007-01-01] Reference

In one Welsh legend the cauldron of the Head of Annwfn has around it a rim of pearls.    From Wordnik.com.  [Celtic Religion in Pre-Christian Times] Reference

Troesom yno a dringo i fyny Glyn Cuch sef yr union fan lle'r oedd Pwyll Pendefig Dyfed wedi'i gael ei hun yn mynd i Annwfn, y byd nad yw'n fyd.    From Wordnik.com.  [Blogiadur.com] Reference

Welsh, in addition to Annwfn, a term which seems to mean the 'Not-world,' we have other names for the world below, such as 'anghar,' the loveless place; 'difant,' the unrimmed place (whence the modern Welsh word.    From Wordnik.com.  [Celtic Religion in Pre-Christian Times] Reference