'Gueith camlann in qua Arthur et Medraut corruerunt.' Thus reads the Annales Cambraie for the year 537.
It translates as 'the strife of Camlann, in which Arthur and Medraut fell'. That latter name is more usually rendered as Modred these days, while Camlan has lost that final 'n'.
That is all that the earliest written documentation says. We don't know who Arthur and Medraut were, and if they were fighting on the same side or against each other. Much later stories filled in details, which might not be quite historically accurate.
But the placename evidence tells its own tale, which you will discover as you wander through this legendary Arthurian landscape.