The last two books in this series are a little out of sequence, since Arthur did die and was taken off to Avalon in the third book. So here in the fifth book, we have the story of the grail, in the middle of one of the times not covered in Arthur.
After the battles covered in Pendragon, Arthur almost died, but Merlin brought him to King Avallach’s palace, where Avallach used the Grail to heal Arthur. At that point, taken over by pride, Arthur decides to build a shrine to the Grail nearby, and naturally, Morgain gets involved to ruin his plans.
She’s had a daughter by her step-son Lot, named Morgaws, and she sends Morgaws in to corrupt the fellowship. She manages to ensnare Llenlleawg (the closest thing to Lancelot in these stories) to her side, and the grail is stolen, along with the Queen. (Again, this the closest you get in this series to Lancelot and Guinevere running off together – in this case, it’s against Gwenhwyvar and the champion’s actual will.) The Cymbrogi must journey into Llyonesse to get the Grail back. It’s a harrowing journey.
This is an interesting way to bring Morgaws/Morgause into the story – it’s always interesting to see how different people interpret Morgan and Morgause – sometimes they’re the same person, or they can be different, but with various relationships to each other. This was a good way to bring the two as separate entities into the story.
It was an interesting interlude, but somewhat short – I think it would have worked better as a section of Arthur.