The Holy Grail is a legendary vessel that is believed to have been present at the Last Supper or the Crucifixion, or possibly both. Quests in search of it have been included in many works of fiction since the 12th century, when legends about this mythical object began to proliferate in England and France. Unlike very real Christian artifacts that have been successfully traced and discovered, the Holy Grail is generally regarded as an entirely mythical object, which exists in the mind rather than in reality.
There are several different versions of the story about the Grail. It is said to be a plate, dish, or cup, and it may be made from ceramic, metal, glass, or crystal, depending on which legend one believes. According to some stories, it was a cup used to serve wine at the Last Supper, so it has an association with Christ and the practice of Communion. In other stories, the Grail was used to capture the blood that flowed from Christ's wounds, while other stories say that the same vessel was used for both purposes.
Supposedly, Joseph of Arimathea took the Grail with him to England, where it was eventually lost. Many legends state that the vessel is still in England, perhaps hidden by supernatural means. In most stories, it is said that someone who is remarkably pure and noble will be able to see the Grail, and quests for it are usually equated with proving nobility and righteousness.
Stories about the Holy Grail began appearing 1,200 years after the birth of Christ, which strongly suggests that these legends were invented after the fact. In the case of real Christian artifacts, stories and legends are present from the time that these artifacts were said to have been used or revealed, providing strong evidence for their actual existence, along with information that can be used to trace the fate of these objects. The Grail stories appear to have been invented by medieval society, with the vessel standing in as a metaphor for purity and the value of questing for a goal.
Some historians have suggested that the legends may have evolved from the sacred cauldron or chalice that is present in the Celtic tradition. According to these historians, early Christians either absorbed the myth and repurposed it, or actively adapted the myth so that reluctant converts would associate Christianity with their beliefs. Christianity certainly has a history of adapting the festivals and beliefs of other religions to make converts more comfortable, so this theory is not entirely implausible.