Nightbringer | The Arthurian Online Encyclopedia

Three Chieftains of Arthur’s Court

Triad no. 9 lists Fflewdwr Fflam Wledic and an unnamned son of Seidi as the chieftains. 

Culhwch gives Chadreith mab Seidi as cheiftain.

See also
Three Sovereigns of Arthur’s Court | The Legend of King Arthur

Red Book of Hergest | C. 1425
Text of the Mabinogion and Other Welsh Tales | John Rhys, 1887
Culhwch and Olwen | Late 11th century