The Holy Grail
I'm not far enough into Sword at Sunset to learn if she manages to work the holy grail into the story, but this thread on RPG.net has some really interesting ideas for working the Holy Grail into a gaming environment, so I thought I'd share:
https://forum.rpg.net/index.php?threads/the-holy-grail-and-sin.835507/

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Comments
I think she sees this as one of the "later importations" and hence not part of her tale. Lancelot is another such, and I suspect that (although she doesn't explicitly say so) she would relegate both of these, plus Galahad and such like, to later strands of the material.
No holy grail, no Lancelot, and no Merlin. Though there is a notable sword, rightfully belonging to the High King.
Lancelot is there, in the form of Bedwyr.
I would definitely say that Bedwyr resembles the traditional Lancelot (aside from the name) more than Bedivere. I think the character's an amalgam.
Bedwyr is far more Lancelot than Bedivere, despite the name.