St. Collen and Gwyn ap Nudd
From Guest's Mabinogion:
A very curious legend, in which Gwyn ab Nudd bears a conspicuous part, is contained in the Life of St. Collen (Buchedd Cohen), which is printed in a collection of Welsh remains, entitled the Greal. This Saint was the son of Gwynawc, ab Caledawc, ab Cawrdav, ab Caradawc Vreichvras, and having distinguished himself greatly in foreign countries by his zeal and piety, he returned to Britain and became Abbot of Glastonbury; after a time Collen desired to lead a life of greater austerity than his high office at Glastonbury permitted; so he departed thence, and went forth to preach to the people. The impiety, however, which he met with distressed him so much, that at length he withdrew to a mountain, "where he made himself a cell under the shelter of a rock, in a remote and secluded spot.
[The translation begins here:]
"And as he was one day in his cell, he heard two men conversing about Gwyn ab Nudd, and saying that he was king of Annwn and of the Fairies. And Collen put his head out of his cell, and said to them, 'Hold your tongues quickly, those are but Devils.'--'Hold thou thy tongue,' said they, 'thou shalt receive a reproof from him.' And Collen shut his cell as before.
"And soon after, he heard a knocking at the door of his cell, and some one inquired if he were within. Then said Collen, 'I am; who is it that asks?’ ‘It is I, a messenger from Gwyn ab Nudd, the king of Annwn, to command thee to come and speak with him on the top of the hill at noon.’
"But Collen did not go. And the next day behold the same messenger came, ordering Collen to go and speak with the king on the top of the hill at noon.
"But Collen did not go. And the third day behold the same messenger came, ordering Collen to go and speak with the king on the top of the hill at noon. ‘And if thou dost not go, Collen, thou wilt be the worst for it.’
"Then Collen, being afraid, arose, and prepared some holy water, and put it in a flask at his side, and went to the top of the hill. And when he came there, he saw the fairest castle he had ever beheld, and around it the best appointed troops, and numbers of minstrels, and every kind of music of voice and string, and steeds with youths upon them the comeliest in the world, and maidens of elegant aspect, sprightly, light of foot, of graceful apparel, and in the bloom of youth and every magnificence becoming the court of a puissant sovereign. And he beheld a courteous man on the top of the castle, who bade him enter, saying that the king was waiting for him to come to meat. And Collen went into the castle, and when he came there, the king was sitting in a golden chair. And he welcomed Collen honourably and desired him to eat, assuring him that, besides what he saw, he should have the most luxurious of every dainty and delicacy that the mind could desire, and should be supplied with every drink and liquor that his heart could wish; and that there should be in readiness for him every luxury of courtesy and service, of banquet and of honourable entertainment, of rank and of presents: and every respect and welcome due to a man of his wisdom.
‘I will not eat the leaves of the trees,’ said Collen. ‘Didst thou ever see men of better equipment than those in red and blue?’ asked the king.
"‘Their equipment is good enough,’ said Collen, ‘for such equipment as it is.’
"‘What kind of equipment is that?’ said the king.
"Then said Collen, ‘The red on the one part signifies burning, and the blue on the other signifies coldness.’ And with that Collen drew out his flask, and threw the holy water on their heads, whereupon they vanished from his sight, so that there was neither castle, nor troops, nor men, nor maidens, nor music, nor song, nor steeds, nor youths, nor banquet, nor the appearance of any thing whatever, but the green hillocks."
This is only a section of the "Buchedd Collin". It is from the endnotes of Guest's Mabinogion translation. (Up until recently, I thought it might be a forgery, until I found the original Welsh text in S. Baring-Gould's Lives of the British Saints.)