This third novel in Newman's Arthurian trilogy has less of the comic fretwork of her other Guinevere tales--not surprising, since there's little jollity to be found in the break-up of Arthur's kingdom and the demise of just about everybody. The cause of nearly all the grief? Modred, of course: Arthur's son by his enchantress sister Morgan Le Fay--who's furious on first hearing of his parentage from Mother. (""That's just wonderful! Do you have any idea what people think about incest?"") Meanwhile, the affair of Guinevere and Lancelot continues, Arthur seemingly unaware; Guinevere happily dotes on Lancelot's son, dear boy Galahad--who'll eventually be lured away by Sir Percival's vision of the Fisher long and the Holy Grail. (Arthur has his doubts about the Grail: ""who will maintain my laws in Britain if all my knights are out hunting phantom tableware?"") But, amid familiar song cues from Lerner & Loewe's Camelot, the royal adultery inevitably brings tragedy: Guinevere is accused of adultery and treachery while Arthur is away; about to be burned at the stake, she's rescued by Lancelot--who winds up killing his dearest friends. And Modred, now Arthur's right-hand man, switches into high gear--bringing on a takeover of the kingdom (and Guinevere), dismemberment of the nation . . . and the final bloody death-battle between father and son. With magical appearances and some feminist updating: a light-weight, airy retelling of the old stories, occasionally amusing.