Gwyn the son of Nudd, his father being sometimes called Nudd Llaw Ereint or Nudd the Silver Hand, and who is compatible with the Irish king Nuada Airgetlam. As such, Gwyn could be synomymous with the Irish Fionn mac Cumhail. Both the words gwyn and fionn have possible meanings of "bright" or "fair".
Gwyn appears in the Arthurian tale of Culhwch and Olwen in the Mabinogion. Here he abducts Creiddylad, the betrothed of Gwythr ap Greidawl. After Arthur's intervention Gwyn and Gwythr both agree to duel each other for the hand of Creiddylad every May Day until Doomsday. Arthur had needed the war between these two to stop, and he'd needed them to assist in the hunt of the wild boar, Twrch Trwyth. Representing the Winter King, Gwyn is doomed to lose every Beltane, but he rises again in time for Samhain.
Entwined with his mythology is that of Glastonbury Tor - thought to be a gateway to his realm of the Underworld. There are many themes that tie him into popular Arthurian legend, which later became christanized. The Celtic Realm of the Dead, the Underworld, is said to contain Ceridwen's Cauldron, the Cauldron of Rebirth. Glastonbury is reputed to be one of the possible hiding places of the Holy Grail, the mythical cup that restores life. The series of terraces on the Tor claimed to be a spiral labyrinth, Caer Arianrhod, the spiral Castle, resides in the heart of the Underworld.
As the son of Nudd, he is also the grandson of Beli Mawr, and nephew to Arianrhod. The realms of the Dead appear to be very much the family business! If Beli and Don are equivalent to the Irish Bile and Danu, then logically the Children of Gwynedd are equivalent to the Tuatha de Danaan, the latter who later became synonymous with People of the Fae.
In both roles he is the Lord of the Wild Hunt, a mantle taken up by Herne and Woden in other parts of the country. He is accompanied by his hounds, the Cwn Annwn, one black, one white and one red, riding through the night gathering the souls of the dead, the fallen warriors, escorting them back to Annwn. The Wild Hunt is traditionally linked with Samhain.
Sources:
Wikipedia
danuforest.co.uk
