Complex sentence: The earliest known reference to the battle of Camlann is an entry in the 10th-century "Annales Cambriae", recording the battle in the year 537 and mention Mordred (Medraut) but do not specify that he and Arthur fought on opposite sides.
Split into simpler sentences: The earliest known reference to the battle of Camlann is an entry in the 10th-century "Annales Cambriae", recording the battle in the year 537. It mentions Mordred (Medraut), but it does not specify that he and Arthur fought on opposite sides.