The story quest begins when you find the tablets in the Deiform quest and give the artifacts to either the Worm or Denholm. The Grail Cycle mission in Tainted Grail: The Fall of Avalon is brief, but we felt it necessary to dedicate a separate guide to it in order to describe the consequences of your decisions.
Regardless of whom you give the tablets to, Lord Iwein will announce that he is setting out on a quest for the Holy Grail. He will therefore ask the hero to gather companions. The party must include the Worm, Erphir, and Neante; to do this, you need to complete a couple of quests: