Third in Penman's ambitious and admirable trilogy (Here Be Dragons, 1985; Falls the Shadow, 1988) centered on the medieval maelstroms of power, passions, and terror during the reigns of England's King John, weak Henry III, and, here, that ``Hammer of the Scots,'' pounder of the Welsh and any others threatening the supremacy of the Crown--Edward I. This is the story of the deadly reckoning, the final defeat of the descendants and allies of Simon de Montfort (whose rebellion against kingly power is chronicled in the first two novels) as well as the tale of the end of an independent Wales. The focus here, much seen through the eyes of a young squire, is on the tactics of two rulers--Edward I of England and his cousinly kin (a genealogical table--a necessity!--will be included) Llewelyn ab Gruffydd, Prince of Wales--judicious, devoted to his land and people, but not one to underestimate Edward's hunger for absolute English supremacy and his ability to achieve it. Around the two, and back and forth, is a motley group: dangerous-to-loyal kin, rumbling nobles and wily clergy, women in love and in terrible grief. Closest to Llewelyn is Ellen, daughter of Simon de Montfort, whose marriage to him is unconsummated for weary months, serving as she does as Edward's lure in a diplomatic game plan. Also, strangely close, is Llewelyn's brother Davydd, a charming, intelligent Cain, betraying again and again--although love can survive. At the close, Davydd's rash action will precipitate the final battle and the deaths of two last Welsh Princes of Wales. Penman's easy narrative and dialogue (except for a ``mayhap'' here and there, mainly 20th-century all-purpose) move the reader effortlessly from chill prisons to the blaze of festivals, the coziness of women's quarters and in the beauty or punishment of capricious winds and weathers. With a large cast of sturdy characters, a gripping (true) story, and research that supports, rather than impales, good storytelling: a first-rate historical novel, crowning a solid series.