The Quest of the Holy Grail tells how Lancelot the Elder left Gaul and came to Britain where he married the daughter of the King of Ireland. This is repeated at the end of the History of the Holy Grail where the account claims that Lancelot held the land that had belonged to his/her father. But nowhere else is there even a hint that Lancelot might have been King of Ireland and so it should probably be understood to mean “his” father.
When Lancelot the elder was murdered by his cousin, the Duke of the White Fortress, the Duke buried his headless body near to where he was slain. When Queen Marche heard of Lancelot the elder’s death, she wished to remove his body from the tomb and rebury it in a nearby chapel, but no man was able to remove the body from the grave and so they erected a tomb above the body.
Queen Marche, before her own death, arranged to be buried in a tomb in front of the altar in the chapel and requested that when, in the future, a knight would come who could take her husband’s body from his tomb, that it would be then laid next to her body in the same tomb. Thus was done. Queen Marche was buried in the tomb in front of the altar and when Lancelot of the Lake came to the White Land, he was able to recover his grandfather’s body. By the advice of a hermit, Lancelot broke open the tomb before the altar and saw the body of his grandmother miraculously uncorrupt. Lancelot placed his grandfather’s head and body next to his grandmother’s body, and then reclosed the tomb.