The oldest surviving mention of Mordred, under the Welsh form Medraut, gives almost nothing away. The Annales Cambriae, a chronicle compiled in the tenth century from earlier material, records for a year usually reckoned as 537 only that the Battle of Camlann took place, in which Arthur and Medraut fell, with no statement of whose side Medraut fought on, what caused the battle, or what relationship, if any, connected the two men. This bare entry is one of the very few pieces of evidence historians who argue for a real figure behind the Arthur legend can point to, and it is exactly this bareness that leaves the door open for every later writer to fill in the story however the developing legend needed: an ally who fell fighting beside Arthur, an enemy who killed him, a usurper, or eventually a son. Geoffrey of Monmouth, writing his Historia Regum Britanniae some six centuries after the annal entry, is the first surviving text to resolve the ambiguity decisively, casting Medraut, in his Latin the more familiar Modredus, as Arthur's own nephew and a straightforward traitor, the shape nearly every later telling of the story would keep even as it added new layers on top of it.