‘Bright Day’
Daughter of Amlawdd, sister of Igerne (Igraine of Tintagil), wife of Cilydd (Kilwydd), and mother of Culhwch. When she became pregnant with Culhwch, Goleuddydd went mad and lived outdoors, aimlessly wandered the countryside, but she returned to sanity before she gave birth. She delivered in the midst of some pigs, and her son was thus name “Culhwch”, or “pig pen”, in Welsh hwch means ‘pigs’.
She was mortally wounded giving birth, and died soon afterwards. Before she died, she made her husband promise not to remarry until he saw a two-headed thorn growing from her grave. She then summoned her confessor and told him to trim the grave every year, so that nothing would grow upon it. The confessor forgot his promise after seven years, and Cilydd remarried.
Later Arthurian legend additionally made Goleuddydd the sister of Igraine and the aunt of King Arthur. Though she is normally given as the sister of Igraine, one genealogy for Arthur unusually makes her the sister of Custennin and Yspaddadden (Ysbadadden), and the aunt of Igraine.
Source
Culhwch and Olwen | Late 11th century