The Vita Merlini or the Life of Merlin was written by Geoffrey of Monmouth around 1150 CE. This may be of particular interest to you Reylari because it deals with Morgen L'Fey in Avalon. Geoffrey calls the Isle "Insula Avalonis", Which he describes as follows:

The island of apples which men call 'The Fortunate Isle' gets its name from the fact that it produces all things of itself; the fields there have no need of the ploughs of the farmers and all cultivation is lacking except what nature provides. Of its own accord it produces grain and grapes, and apple trees grow in its wood from the close-clipped grass. The ground of its own accord produces everything instead of merely grass, and people live there a hundred years or more.

There nine sisters rule by a pleasing set of laws those who come to them from our country. She who is first of them is more skilled in the healing art and excels her sisters in the beauty of her person. Morgen is her name, and she has learned what useful properties all the herbs contain, so that she can cure sick bodies. She also knows an art by which to change her shape, and to cleave the air on new wings like Daedalus; when she wishes she is at Brest, Chartres, or Pavia, and when she wills she slips down from the air onto your shores.

The interesting thing to note here is that Geoffrey must have had some prior knowledge about the "Preddiu Annwn" as he relates the same story of Merlin taking the wounded King Arthur to the Island for healing. The following passage will exeplify it.

Thither after the battle of Camlan we took the wounded Arthur, guided by Barinthus to whom the waters and the stars of heaven were well known. With him steering the ship we arrive there with the prince, Morgen received us with fitting honor, and in her chamber she placed the king on a golden bed and with her own hand she uncovered his honorable wound and gazed at it for a long time. At length she said that health could be restored to him if he stayed with her for a long time and made use of her healing art. Rejoicing, therefore, we entrusted the king to her and returning spread our sails to the favoring winds.

I just thought it would be interesting to note the different legends concerning Arthur and Avalon.