The tomb is at least 3,500 years older than the legend of King Arthur, whose stories took place in the 5th and 6th centuries and were first mentioned centuries later. In more modern times, it was believed that the site served as the inspiration for the Stone Table in the C.S. Lewis book, The Lion, the Witch, and the Wardrobe.
Article source: https://www.huffingtonpost.co.uk/entry/king-arthur-legend-tomb-excavation_uk_62ce9754e4b0942327826708