This legend attached to this dolmen burial site is quite simply invented in the middle ages. King Henry VIIIs Antiquary, John Leland, believed he found the last battle of King Arthur here because of the resemblance of the name of the battle Camlan to the nearby River Camel and town Camelford. It stuck, as romantic and incorrect stories tend to do, and for five centuries legend has built up around the whole site with regards to King Arthur and the last battle here.
There is a Roman inscription on the stone that bears the legendary title of King Arthur's Tomb. Unfortunately for Arthur it only references a poor fellow named Latinus. In 1586 the topographer William Camden wrote about the site and reported that artifacts were dug up that must be the proof it was Arthur's Tomb. But it was probably from troops who fought for King Egbert of Wessex in 825 when he took Cornwall.
The tomb is not Arthur's, but the legend is over 500 years old and so deserves its place in history nonetheless.