The Holy Grail is supposedly the cup or dish Christ used at the Last Supper that was later used by Joseph of Arimathea to catch Christ’s blood as he hung on the cross. It is mythical and not accepted as true by any church. The stories about it were written between 1170 AD and 1240 AD. Most are in French although some are in English and German. They are a part of the legends of King Arthur.
It’s supposed to have miraculous properties and people have to be spiritually mature to partake of it’s grace. You can see where it gets a little off track from true Christian teachings.
It’s like the stories of the “three wise men.” They’re entertaining but have nothing to do with the Bible which doesn’t mention a number of wise men or any details other than the short visit with Herod and the gifts brought. The same with the Holy Grail. We know Christ used a cup at the last supper and no more.