Creiddylad first appears in Culhwch and Olwen within the immense list of people invoked at Arthur's court. The list groups her among the gold-torqued women of the island and names her as the daughter of Lludd Llaw Eraint. It describes her in unusually elevated terms as the maiden of greatest majesty in the Three Islands of Britain and their neighbouring islands. The passage establishes her fame and parentage but gives her no speech, action or independent adventure.

The tale later explains the feud attached to her name. In some English translations Creiddylad and Gwythyr son of Greidawl are described as betrothed; closer renderings say that she went with Gwythyr and that before he slept with her, Gwyn ap Nudd came and took her by force. Either way, the central action is clear: Gwyn removes Creiddylad from Gwythyr, and Gwythyr raises a host to recover her.

Gwyn defeats the opposing force and captures several of Gwythyr's companions, including Greid son of Eri and other named warriors. The episode becomes especially brutal when Nwython is killed and his son Cyledr is forced to eat his father's heart, driving Cyledr mad. Creiddylad herself remains offstage during the violence. The story focuses on what the rival men do around her rather than on any recorded response from her.

Arthur hears of the conflict, comes north and forces a settlement. He frees Gwyn's prisoners and orders that Creiddylad remain untouched in her father's house, with neither rival gaining advantage. From then until Judgement Day, Gwyn and Gwythyr must fight each other every first of May. The man who wins on the final day will receive Creiddylad. This perpetual contest is the most distinctive part of her surviving story.

The Black Book of Carmarthen preserves another important connection when Gwyn identifies himself as the lover of Creurdilad, a form of Creiddylad's name. That poem confirms her importance to Gwyn outside the prose feud in Culhwch and Olwen. It does not, however, turn Creiddylad into a fully narrated goddess with an independent seasonal cult.

Modern interpreters have often read the May Day combat as a seasonal contest between winter and summer and have called Creiddylad a flower bride or sovereignty figure. Those interpretations can be useful comparative readings, but the medieval source itself does not call her a flower goddess, identify Gwyn and Gwythyr as literal seasons or explain the annual fight through a pagan ritual. Mythopouch therefore keeps the source narrative separate from later symbolic interpretation.

Creiddylad's father should also be handled carefully. Culhwch and Olwen names him as Lludd Llaw Eraint, the Silver-Handed Lludd. That figure is often merged in modern mythography with Lludd, son of Beli Mawr, from Lludd and Llefelys, but P. C. Bartrum treats the traditions separately. This record therefore preserves Creiddylad's actual medieval patronymic without automatically making her the daughter of the king in the Three Plagues tale.