As Cecil explains in the original column, the Grail first appears in the last of the Arthurian poems of Chrétien de Troyes, who was also the inventor of Sir Lancelot, the Queen’s lover. In Chrétien’s unfinished poem, Perceval, le Conte du Graal (ca. 1181-1190), it is simply described as “un graal”, a relatively obscure Old French word for dish. There appear to be some old Celtic legends behind it all, but Chrétien essentially made a new story of it, just as Wagner’s “Ring” operas are heavily based on ancient myths, the Völsunga saga, and the Niebelungenlied, but ultimately go off in a completely different direction.
The story was wildly popular, and several other poets wrote continuations and attempted endings of it, none of them quite definitive, because no one quite knew where it was supposed to go. So it continued to bubble, and, as continuations became more and more impractical, poets started instead to rewrite the story from the beginning.
In Chrétien’s version, the main hero is Percival, who became Parzifal or Parsifal in German. Wolfgang von Eschenbach’s poem became the main version in the German-speaking world, and, eventually, the inspiration for another Wagner opera.
But in France, where most of the Arthurian literature developed, the main line of development passed through Robert de Boron. It was his version that nailed down the idea that the Grail was the cup of the Last Supper. (It was also he who nailed down the story of Merlin, and permanently attached him to Arthur’s court.)
The final stage came in a long prose version of the entire story of Arthur. Like most medieval works, it doesn’t have a title; unlike most, it’s never receive a de-facto title, either. It is variously referred to as “The Lancelot-Grail”, “The Prose Lancelot”, “The Vulgate Cycle”, or “The Pseudo-Map Cycle” (because it claims to have been written by Walter Map, which it certainly was not). It established two vital parts of the story.
First, although Chrétien had introduced the figure of Lancelot as Guinevere’s lover, and a mighty knight, the general tendency until then was that whoever was the hero of the current story was King Arthur’s greatest knight. The Vulgate Cycle established Lancelot permanently in that role.
Second, Percival got demoted. Instead of being the hero of the Grail adventure, he became one of three. Sir Bors, the ordinary man, Sir Percival, the innocent, and a brand-new character, Sir Galahad, the mystic. And then, in a brilliant stroke, the two main plots were tied together by making Sir Galahad the bastard son of Sir Lancelot, begotten by him on a young woman who had been enchanted to look like the Queen.
The Vulgate Cycle is long (about a million and half words), and was only translated into English for the first time a few years ago, in five oversize volumes from Garland Press (which means you’ll mostly find it in university libraries). But it was very popular in French. Pretty much every version of the Arthur story written since has been based on either it, or on the ancient Welsh and Breton legends. This includes Sir Thomas Mallory’s Le Morte Darthur, which is essentially a “Reader’s Digest” version of the Vulgate, although it also adds the story of Trystram and Isolde.