One of the party who were to help Culhwch locate and win the hand of the maiden Olwen, daughter of the chief giant Yspaddaden (Ysbadadden).
Arthur says he is
as good a guide in lands he had never seen as he was in his own country.
The other members of the party were Cei (Kay), Bedwyr (Bedivere), Gwrhyr, Gwalchmai fab Gwyar and Menw fab Teirgwaedd. Each member of the party was chosen for a particular skill, that of Cynddylig being obvious from his epithet.
Elidyr Gyvarwydd | The Legend of King Arthur
Culhwch and Olwen | Late 11th century