Three Exalted 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) and Mabon (Madoc) the son of Modron, 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.
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.