What is "cilydd"

Cilydd is the father of Culhwch, hero of the Middle Welsh prose tale Culhwch and Olwen, the earliest of the collection usually referred to as the Mabinogion. He is the son of Prince Celyddon, husband to Goleuddydd, who bore him his son. When Goleuddydd dies after giving birth to Culhwch, Cilydd hears about another woman, the wife of King Doged. Cilydd slays Doged and procures his lands and family, marrying the widow, who becomes Culhwch's hated stepmother.