An old, medieval legend.  In its original version, never finished, so others added and expanded it to give a number of different versions.  I first came across it as the Terry Gilliam film, with Robin Williams.  And now I realise Amfortas, a character in the opera Parsifal by Wagner that I saw years ago, is the same person.

As I understand it , the story goes like this.  Parsifal, the good knight, is travelling late in the day when he comes across a man fishing in a boat.  The man directs him to a secret castle nearby for the night.  When Parsifal arrives, the fisherman reveals himself as the king, but he is wounded, with a wound that will not heal.  So he cannot walk, or hunt, or ride, or do any of the noble pursuits that a king should – so instead, he secretly goes fishing.

A banquet is held, during which a strange procession of a bloodstained lance, candelabras, and an ornate dish [called a grail, although we usually think of grail meaning a chalice now].  The king takes a wafer from the grail, which heals his wound, albeit temporarily.  He does not say anything about all this, because his teacher taught him never to ask questions about things he does not understand.  In the morning, Parsifal wakes to find himself alone.  He sets off on his journey, but meets a maiden who he tells of his experience.  She asks him if he asked "Whom does the grail serve?"  He did not.  "Only then will the king be healed".  Parsifal tries to make his way back to the castle but cannot find it.

In other versions, the grail is the cup Christ used at the last Supper, and Amfortas is its guardian.  The land is barren, because its king is maimed and infertile himself.  Parsifal eventually finds his way back, and heals the king.  Parsifal eventually succeeds the king.

There are some old celtic ideas that may feed into the story, but what is notable is how at the time of the failed crusades, people turned away from the classical traditions of the Mediterranean and near East, and instead harked back to an earlier, mystical culture with its origins in the far north west of Europe.  Part of the fascination with the Grail was probably due to the loss of Jerusalem and the holy sites to the Saracens.