Gwenddoleu ap Ceidio died at the Battle of Arfderydd, a battle with no written record.
The Battle of Arfderydd (also known as Arderydd) was fought, according to the Annales Cambriae, in 573. Annales Cambriae (Latin for The Annals of Wales) is the title given to a complex of Cambro-Latin chronicles compiled or derived from diverse sources at St David's in Dyfed, Wales. The earliest is a 12th-century presumed copy of a mid-10th century original; later editions were compiled in the 13th century. 
