To the Great Deep - A Merlin Mystery, #6

As Guinevere is being led to the stake for treason and adultery, Sir Lancelot leads an army of his supporters to save her from the flames, leaving several of Arthur's knights dead in his wake, including Sir Gawain's favorite brother Gareth. Gawain goads Arthur into a war of vengeance with Lancelot. But Merlin, examining the bodies on the battlefield, realizes that Gareth was killed not by Lancelot's mounted army but by someone on the ground who attacked them from behind.
Once again it is up to Merlin and Gildas to find the real killer of Sir Gareth before Arthur's reign is brought down completely by warring knights and by the machinations of Mordred, who has been left behind to rule in the king's stead.