In Arthur's World, an alternate reality in which King Arthur actually existed, the Battle of Camlann was the final conflict between the forces of King Arthur and those of the would-be usurper, Mordred. The battle was fought near the rural English village of Carbury, not far from Lake Vortigern. The site was rediscovered in the 20th century and excavated by archaeologist Peter Warmsly.
Arthur was mortally wounded at the battle. Merlin, the king's enchanter — an incarnation of the Doctor — placed his body and the sword Excalibur in an interdimensional spaceship and hid it at the bottom of Lake Vortigern.
After the battle, rumours and legends spread that Arthur had not died at the battle, but was being kept in stasis until England needed him again. These stories may have originated with Merlin himself who, in a note to his past self left with Arthur's body, dismissed them as "propaganda".
Behind the scenes
Most Arthurian traditions have Mordred also dying at the battle of Camlann, though some have him survive the battle to die later at Sir Lancelot's hand.