King Henry II heard the story about the legendary ruler's grave on the grounds of Glastonbury Abbey from a Welsh bard.

An excerpt from the article 16 facts about Glastonbury

Henry II shared this news with the prior of the abbey. When the monastery was rebuilt after the fire in 1184, the grave was also searched for. About 2 m underground, a stone slab and a lead cross were found with the inscription: "Here, on the Isle of Avalon, was buried the famous King Arthur." Below this stone slab was found a coffin made of a carved log, containing the bones of a large man with a damaged skull and the bones of a smaller person, identified as Guinevere thanks to a lock of light hair found near the remains.