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The Buchedd Collin, a Welsh life of St Collen dating from 1536, shows the extent to which Annwn was synonymous with fairyland and Gwyn ap Nudd with the king of the fairies in late medieval Wales. Although this particular story was recorded on the eve of the Reformation, it may well draw on earlier tales: As [St Collen] was in his cell one day, he heard two men talking about Gwyn ap Nudd, and saying that he was the King of Annwn (the Under-World) and the Fairies. Collen put his head out, and told them to hold their peace, and those were only demons. They told him to hold his peace, and, besides, he would have to meet Gwyn face to face. By-and-by Collen heard a knocking at his door, and in answer got the reply, ‘It is I, the messenger of Gwyn ap Nudd, King of Annwn, bidding you to come to speak with him on the top of the hill by mid-day’. The saint persistently refused to go day after day, until at last he was threatened with the words, ‘If you don’t come, Collen, it will be the worse for you’. This disconcerted him, and, taking some holy water with him, he went. On reaching the place, Collen beheld there the most beautiful castle that he had ever seen, with the best-appointed troops; a great number of musicians with all manner of instruments; horses with young men riding them; handsome, sprightly maidens, and everything that became the court of a sumptuous king. When Collen entered, he found the king sitting in a chair of gold. Collen was welcomed by him, and asked to seat himself at the table to eat, adding that beside what he saw thereon, he should have the rarest of all dainties, and plenty of every kind of drink. Collen said, ‘I will not eat the tree-leaves’. ‘Hast thou ever’. asked the king, ‘seen men better dressed than these in red and blue?’ Collen said, ‘Their dress is good enough, for such kind as it is’. ‘What kind is that?’ asked the king. Collen said that the red on the one side meant burning, and the other, cold. Then he sprinkled holy water over them, and they all vanished, leaving behind them nothing but green tumps.
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Francis Young (Twilight of the Godlings: The Shadowy Beginnings of Britain's Supernatural Beings)