EL'UNICORNIO, the original 1965 Spanish title of Manuel Mujica Lainez's novel, is a tale for medievalists, Crusade buffs, and folklore fans. It is recounted, with Gallic style and wit, by a forlorn French fairy, M,elusine. Her legend was written down in the 15th century by Jean d'Arras, and her image appeared even earlier in the Duc de Berry's TrMes Riches Heures: the Limbourgs painted her as a tiny golden dragon flying over the chateau of Lusignan, of which she was the supposed architect, builder and first chatelaine. To this day she is revered in Poitou as the region's patroness; historian Claude Marks says "it is impossible to travel in Western or Southwestern France without hearing many references to M,elusine."

References, but not till now the whole story. Manuel Mujica Lainez, a distinguished Argentinian novelist and winner of many prizes (and the high regard of Jorge Luis Borges, who provided a foreword), has chosen to be the amanuensis for M,elusine's memoirs. The tale begins with a brisk summary of the classic legend: once a beautiful Hibernian fairy princess, M,elusine was condemned by her mother's curse and her husband's curiosity to onerous immortality in a semi-serpent form; she is, however, invisible except at the death of a Lusignan prince, which she announces by soaring and shrieking over the castle towers. But, Lainez tells us, she is not a dragon, siren, undine, or banal mermaid; she--and he--concedes the snaky tail and batwings, yes, but they are a dainty silver and blue (the Lusignan colors) and she still boasts white breasts and long chestnut hair. And whether bonne bourgeoise deploring her involuntary soaring and shrieking as theatrical and tasteless, or romantic jeune fille in love again at first sight, this Melusine is wholly French, human and female, proper, passionate, and wise.

One summer night in 1174 A.D. M,elusine is perched in her belltower mourning her long-dead husband and their sons for whom she had built castles throughout Poitou, and over whose sleeping forms she once invisibly wept, like the banished adulteress in Victorian melodrama. She is also doing some research in tomes from the castle library. This is her hobby and her major flaw, since it erupts sometimes in pedantic disquisitions on genealogy or arcane symbolism. Thus the reader is as glad as she when an ox-cart trundles by below bearing a crew of itinerant miracle-play actors--and a very handsome young stonemason. By his one blue and one gold eye, M,elusine recognizes him as yet another, if unacknowledged, Lusignan heir, one of her own descendants. Since he is more needy and much more charming than the incumbent lords of Lusignan, she hops onto the ox- cart.

Soon M,elusine is the invisible witness to a reunion between her prot,eg,e, Aiol, and his long-lost father, a knight-errant who has returned, ruined, from the Crusades and has come to Lusignan to find his son. Aiol vows to accompany his father back to Palestine in search of the Holy Lance. This quest first takes them all on a tour of France, which occasions word pictures as colorful as those in a Book of Hours and which provides a graceful interaction with the social conventions and institutions of the late Middle Ages. In Poitiers, Aiol's sister is spectacularly exorcised in Notre Dame La Grande. In the forest of Lussac they are welcomed by a saintly hermit and his pious cats, and at a castle in the Pyrenees they are feted by the beautiful Lady Seramonde. Modeled on the court of Eleanor of Aquitaine, Castel- Rousillon appears "a microcosm of civilization." But Seramonde's seduction of Aiol enflames the jealous rage not only of invisible, infatuated M,elusine but also of the lady's husband and her troubador. The catastrophe is barbaric, and the villain turns into a werewolf.

During a tournament in Champagne, Aiol's father is hewn down by two evil cousins, but Aiol is finally knighted. M,elusine herself is also suddenly transformed into a handsome knight, her malicious mother's answer to her lovesick prayers that she be made young, visible, and beautiful again. Frustrated but faithful, as Melusin of Pleurs, she/he sets sail with Aiol to fight for King Baldwin and seek the Holy Lance. The crusaders' Palestine is shown in vivid if confusing scenes of luxury and squalor, faith and intrigue, and family rivalries. Just as the story's most savage event takes place in a castle proud of its culture and chivalry, so the most inspiring and poignant action occurs on a battlefield: young King Baldwin, blind and near death with leprosy, endures an excruciating trek across Palestine to save besieged Kerak, and says simply: "You sent to us. We are here."

Myths, legends, and fables often are revived for political purposes, as Anouilh did with Antigone. But this gorgeous, rueful, and charming historical fantasy is more like the fairy tales of Giraudoux; it offers us no message other than the glory and grief of human history, and the persistence of human passions. The lone survivor of the quest and restored to her hybrid form, M,elusine flies back to Lusignan to mourn Aiol and to compose her biography. But she never explains why she had stayed around Lusignan, brooding and researching, all those years till Aiol came by. After all, it's just a short flight, even by batwing, to Poitiers, which during those decades was the seat of Eleanor's brilliant court. Melusine never mentions this--d she resent that great queen as a rival? But perhaps she peeked in from time to time. It seems significant that she rode off on the ox- cart in the summer of 1174: that very spring Eleanor had been put in prison by Henry, and her daughter Marie had presided over the last reunion of the Court of Love. Romance and chivalry were gone from Poitou.