Gwenddydd, also known as Gwendydd and Ganieda, is a character from Welsh legend. She first appears in the early Welsh Myrddin poems and in the 12th-century Latin Vita Merlini by Geoffrey of Monmouth, where she is represented as being a figure in the Old North of Britain, the sister of Myrddin or Merlin, and a prophet in her own right. Geoffrey also makes her the wife of the northern king Rhydderch Hael. She was remembered in Welsh traditions recorded in the 16th century by Elis Gruffydd, and even as late as the 18th century. Since the late 19th century she has occasionally appeared as Merlin's sister or lover in Arthurian fiction, poetry and drama by writers such as Laurence Binyon, John Cowper Powys, John Arden, Margaretta D'Arcy and Stephen R. Lawhead.