Gwyn ap Nudd first appears in the Black Book of Carmarthen, one of the four great works of medieval Welsh literature, compiled around 1250 but containing poetry from as early as the 9th century. In Poem XXXIV, he is addressed in a dialogue by Gwyddno Garanhir, a legendary king of the drowned kingdom of Cantre'r Gwaelod. Gwyn describes himself as a warrior of terrifying aspect: "Bull of battle, fierce, terrible, ruler of a host, unpleasant in wrath." He rides a white horse called Carngrwn, Round-hooved, with a golden saddle, and wears an engraved ring. His face is blackened, a mark of his otherworldly nature, and his finest hound is Dormarch, one of the Cŵn Annwn, the Hounds of the Otherworld. In the poem, Gwyn reveals his role as a psychopomp, a guide of souls, gathering the spirits of fallen British warriors from the field of slaughter and leading them to Annwn, the not-world that lies beyond mortal perception. He calls himself the lover of Creiddylad, the daughter of Lludd Llaw Ereint, binding his identity to the maiden whose abduction would become the central conflict of his legend.

The fullest account of Gwyn's deeds appears in Culhwch and Olwen, the oldest Arthurian tale, preserved in the White Book of Rhydderch and the Red Book of Hergest from the 14th century but composed centuries earlier. Culhwch ap Cilydd, a young prince cursed by his stepmother, must win the hand of Olwen, the daughter of the giant Ysbaddaden, by completing a series of impossible tasks. One of these tasks is to hunt Twrch Trwyth, a king who had been transformed into a monstrous boar by God, and to retrieve the comb and scissors from the beast's head. Arthur, the king of Britain, assembles a vast retinue of warriors to aid Culhwch, and among them is Gwyn ap Nudd, described as the man "in whom God put the spirit of the demons of Annwn, lest the world be destroyed." He is the only huntsman who can ride Du y Moroedd, the Black of the Seas, the legendary water horse capable of pursuing Twrch Trwyth into the ocean. Without Gwyn's aid, the hunt is impossible, and his presence marks him as both indispensable and dangerous, a force of the Otherworld bound to the service of the mortal king.

But Gwyn's role in Culhwch and Olwen is not limited to the hunt. He had abducted Creiddylad, the daughter of Lludd Llaw Ereint, from her betrothed, Gwythyr ap Greidawl, a warrior whose name means Victor Son of Scorcher. Gwythyr raised a great host against Gwyn, and the two fought a vicious battle. Gwyn was victorious, and in the aftermath he committed an act of such cruelty that it shocked even the warriors of Arthur's court. He murdered Nwython, one of Gwythyr's noblemen, and forced Nwython's son, Cyledr, to eat his father's heart. The torture drove Cyledr mad, and he earned the epithet Wyllt, the Wild. Arthur intervened, commanding Gwyn to release his captives and decreeing that Gwyn and Gwythyr must fight for Creiddylad every May Day, Calan Mai, until Judgement Day. The victor on that final day would claim the maiden. This perpetual combat, echoing older seasonal myths of winter and summer, binds Gwyn to an endless cycle of conflict, a lord of the Otherworld who can never rest.

In the same tale, Gwyn and Gwythyr advise Arthur in his quest to find Pennant Gofid, the Uplands of Hell, and to defeat the Hag of Pennant Gofid, a terrifying otherworldly witch. Their advice is cryptic and their assistance limited, suggesting that Gwyn serves Arthur only when it suits him, and that his loyalty is to Annwn rather than to Camelot. This ambiguity is central to Gwyn's character. He is not evil, but he is not benevolent either. He is other, a being whose morality and motives lie beyond human comprehension. The 14th-century Latin manuscript Speculum Christiani records Welsh soothsayers invoking him with the words: "To the king of Spirits, and to his queen -- Gwyn ap Nudd, you who are yonder in the forest, for love of your mate, permit us to enter your dwelling." This invocation shows that Gwyn was not merely a literary figure but a presence in the folk religion of medieval Wales, a spirit to be petitioned and propitiated.

The later medieval tradition transformed Gwyn from a warrior of Annwn into the king of the Tylwyth Teg, the fair folk or fairies of Wales. In the 16th-century hagiographic text Buchedd Collen, the Life of Saint Collen, the saint is invited to an audience with Gwyn ap Nudd at Glastonbury Tor in Somerset. Collen ascends the hill, falls asleep upon an egg-shaped stone, and awakens inside a magnificent castle where Gwyn sits upon a golden throne surrounded by beautiful maidens, soldiers, and minstrels. The king invites Collen to dine, but the saint refuses, knowing that to accept fairy food is to be trapped in the Otherworld forever. Instead, Collen draws a flask of holy water and casts it upon Gwyn and his court. The castle, the music, and the fairies vanish in an instant, and Collen finds himself alone on the tor. The tale reflects the Christian anxiety about the old pagan powers, but it also preserves the belief that Gwyn's kingdom lies just beneath the surface of the mortal world, accessible through certain hills and tors on certain days. The 14th-century Welsh poet Dafydd ap Gwilym refers to Gwyn in several poems, calling the owl the "fowl of Gwyn ap Nudd" and describing a lake as "the fish lake of Gwyn ap Nudd" and "the palace of the elves and their children." These references show that Gwyn had become a byword for the fairy realm itself, the sovereign of a world that runs parallel to our own, close enough to touch but dangerous to enter.