“Enemy of God” picks up where “The Winter King” left off; Britain is at peace: the Saxons are at bay, and Arthur rules. But disaster, in the form of Guinevere’s ambition and King Mordred’s coming-of-age, is about to strike. Narrator Derfel accompanies Merlin on a quest for a legendary cauldron and the story of Tristan and Iseult is told in an especially brutal and touching way. The conflict between Christianity and paganism provides yet another plot element in this, the second of Cornwell’s “Warlord Chronicles” trilogy. It’s as complex as its predecessor, and just as absorbing.