The Holy Grail is another extremely famous artefact that has real-world inspirations, according to most traditions the Holy Grail was one of the cup's used by Jesus during the Last Supper and like many religious relics is said to have miracolous powers that often vary depending on the witness (in the case of the real-life artefact) or the writer (in the case of fiction).
The search for the Holy Grail was one of the most important plots in medieval romance, especially the tales of King Arthur and throughout history many have been said to have found and hidden the Holy Grail - though as of yet their is no evidence of anyone having ever been in possession of the item (if it even existed at all).