The Holy Grail was a mystical artifact that was a major focus of King Arthur.
Arthur, Lancelot, and Robin decide to go on a quest to find the grail. After getting advice from Merlin on its whereabouts, the group go to Mount Dune Tah Too Tae, where they face off against the Druids and eventually win. They locate the grail inside the Druid King's crotch upon him dying. However, Arthur decides to destroy the Holy Grail. Eventually, it is used by Lancelot in the final battle against Mordred; he collected it from the Druid King's crotch after killing him and destroyed Mordred.
The Holy Grail is the cup that Jesus used to serve the grape juice in at the Last Supper.
For unexplained reasons, the Holy Grail was hidden in the Druid King's crotch. This was until he was killed by Arthur; when he died, his body disappeared and left only his clothes and the Grail. As its powers are unmatched, it was the only thing that could kill Mordred.