Wavering between four and five stars on this one, but giving it five because of the excellence of the writing and the depth of the characterizations (with one possible exception, which I'll get to).
This is a very dark book, about the bonds of love and jealousy within a family. That family, of course, is Arthur's. The main character and narrator is a young man called Medraut. The illegitimate eldest son of the high king, Medraut is bitterly jealous of his younger siblings, especially his younger brother, Lleu. Medraut's name means marksman, which is appropriate because he is a skilled archer who loves the hunt. Frail, timorous Lleu is named for a mythical figure, the bright one.
Medraut loves and is loyal to his father, but his loyalty is tested when the woman he calls godmother visits the court. Morguase is strong-willed, selfish, and highly skilled with poisons. She begins poisoning the young prince of Britain, to the horror of his twin sister and older brother. They attempt to save Lleu on their own, but things soon spiral out of Medraut's control. Even when Morguase has left Camlan, her baneful influence remains.
Which, btw, brings me to the only weak point of this book. Morguase. In a book that is so psychologically astute, how can readers accept a character so purely villainous? We understand Medraut's actions, Artos's, the acts of the young twins Goewin and Lleu, and even those of their cousins, Morguase's sons. While I was reading, I had no trouble believing in Morguase's villainy, too. It was only after I finished (a reread for me) that I began to ask myself why, in this court of flawed, talented, passionate human beings, one character should be purely villainous.
But I never asked myself that while I was reading. As an adult reader, I may want to know what drives Morguase and what twisted her into the monster she seems to be. But I can accept that monsters exist. Among other things, this is an examination of one young man's struggle to transcend the abuse he's been subjected too, rather than passing it on. We can surmise that Morguase, who abuses all her sons psychologically and who torments Medraut physically as well, must herself have been an abuse survivor.
Medraut certainly is, and so, by the end of the tale, is Lleu. Their final confrontation takes place in the dark of the year, between Christmas and New Year's. And it is riveting. This isn't a book I would care to reread too often; it is too dark and disturbing. But it is well worth reading and rereading. A really unique book by a fine writer.
(BTW, for fans of historical fiction, post-Roman Britain is very well evoked. "The Winter Prince" reminds me, in this regard of Sutcliff's "The Lantern Bearers".)