The Three Supreme Prisoners of the Island of Britain
- Llyr Llediaith, in the prison of Euroswydd Wledig,
(Lady Guest believes this name to probably be Ostorius, the Roman commander; another triad says that this imprisonment also lead Bran and his son Caradawg ap Bran--the historical Corocticus--to be imprisoned in Rome, and that it was through them that Christianity then came to the island.)
- and Mabon (Madoc) the son of Modron,
This would be Mabon ap Modron, the Welsh god of youth and rebirth, whose imprisonment and release plays a significant role in Culhwch ac Olwen; see his w/u.)
- and Geyr (Geiryoed) the son of Geyrybed,
and one more exalted than the three, and that was Arthur,
who was for three nights in the Castle of Oeth and Anoeth,
and three nights in the prison of Wen Pendragon,
and three nights in the dark prison under the stone -
And one youth released him from these three prisons; that youth was Goreu the son of Custennin, his cousin.
From "Celtic Twilight
": "The Castle of Oeth and Anoeth is spoken of in the Mabinogion and in another series of the Triads it is named as the prison of the above-mentioned Geyr. In this version, Arthur is not alluded to, but all the members of the families of the other prisoners are said to have shared their captivity, which is designated as the most complete ever known to have taken place."
That Arthur would be imprisoned at a place named "White Head-dragon" (Wen Pendragon) would seem odd, unless one takes into account the story of Merlin and the Dragons, wherein the Red Dragon represents the Britons, and the White Dragon represents the Saxons. Hense, the "White Head-dragon" could refer to Arthur being imprisoned by a Saxon chief, until rescued by a cousin. On the other hand, the White Dragon could be symbolic of evil forces, as the other prisons mentioned are thought to represent Otherworld tourture chambers.
Oeth Anoeth, in the Iolo Mss., is said to have been made by Manawyddan ap Llyr out of bones. This may have been an invention of Iolo Morgannwg, or he may have been recording actual tradition; it's always hard to say with Iolo.
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