Knights with the Grail Vow are Immune to Psychology, and cannot be joined by characters that have The Knight's Vow or The Questing Vow.

Any model with the Grail Vow always starts the game with The Blessing of the Lady, regardless of whether or not the Bretonnian army prays. All attacks made by models with the Grail Vow count as magical, though this does not affect the Knight's mount.