IN TWO EARLIER volumes, The Crystal cave and The Hollow Hills , Mary Stewart has whetted readers' appetites for Arthurian marvels. This final volume should be the climax, and if some of her fans find it an anticlimax, part of the problem lies in the material and part in unrealistic expectations.

We all know the stone, the quest of the Holy Grail, the bittersweet love of Lancelot and Guinevere, the marvelous adventures of knights-errant who' sat with the king around the famous round table when they weren't galloping off to encounter giants, dragons, dread enchanters and the foul types who specialized in abducting maidens. It has been told for us gloriously by Sir Thomas Malory, and by hundreds of adapters after them.

Now, having used two long, exciting novels to get Arthur on the throne, Mary Stewart has reached the final volume of her trilogy and we can settle back expecting to hear the old stories told again with her unique touch.

There is only one trouble with this expectation;Mary Stewart does not fulfill it, and she quite clearly never had any intention of fulfilling it. Her story is not strictly about Arthur but about Merlin, Arthur's prophet and wizard. And when she does get to Arthur, she introduces us to a relative stranger.

The Arthur we have known and loved is a man basically of the 12th century. Mary Stewart ignores him, brushes away as irrelevant all the details with which his legend became encrusted during the age of chivalry and tries to get back to the original Arthur, leader of battles (dux bellorum) who led the bitter struggle 15 centuries ago to maintain the Celtic hegemony against the Germanic invaders of Britain.

The name of Lancelot does not appear in this story; the Holy Grail, the round table, the building of Camelot are dismissed in passing with perfunctory lip service. The colorful adventures of kngiths errant are simply thrown out - quite rightly, since they have nothing to do with the fifth-century leader of battles.

Fair enough; history is history, and Mary Stewart has made a fair stab at putting it back in perspective. Her effort should be recognized even by those who will then go back and reread their Malory. But even for those who will not miss the colorful trimmings usually associated with Arthur, this final volume presents special problems. Strictly speaking, once Arthur is safely on the throne, as he is when this volume opens, Merlin's life work is over. He spends most of The Last Enchantment fading away as gracefully as he can manage. His supernatural power grow fitful, fade and disappear. He is no longer at the center of the action, making things happen, but on the sidelines, observing and reacting, sometimes watching powerless and horrified as the dire prophecies he made in earlier volumes finally come to pass.

But Stewart's subject is Merlin, even in decline, so the role of Arthur is this volume is fitful and erratic; he is a powerful presence but not the central character. He fulfills his destiny in this volume, winning 12 epic battles against the Saxons, subduing the unruly ambitions of the minor British Kings who are his own presumed allies, establishing peace and order in Britain and building Camelot.All this is included, but from Merlin's point of view; the battles are dutifully mentioned but not described, since Merlin is elsewhere when they happen (and, historically, nothing is known of them beyond their names and the fact that Arthur won them).

In the faithful working out of her grand plan, Mary Stewart comes to the most difficult part in this final volume, for it is here that the source material on Merlin becomes hard to handle, chaotic and confusing. Making sense of this material is no easy task, and in this volume one must admire the ingenuity of Stewart's effort more than the plausibility of th estory.

Perhaps the most awkward of these legends is the one of Merlin's infatuation with Nimue, a follower of the Lady of the Lake, to whom he taught his craft and who rewarded him by shutting him up alive inside a mountain where he would die "a shameful death." Unable to accept this ridiculous end to her hero's life, Stewart transforms the legend into a touching love story - the old enchanter reaching fulfillment in his final years, after his great dynastic task has been accomplished, and passing on his gift to his newfound love. Also well-handled is the disturbing story of the slaughter of the innocents - the massacre of newborn children in a vain attempt to locate and eliminate the infant Mordred.

The Grail, the abduction of Guinevere and other traditional materials are also presented, but the treatment is Stewart's own, the emphasis shifted for her purpose, which is not simply to recast old material but to bring alive a long-dead historical epoch - not the Middle Ages of Malory but the Dark Ages of the original Arthur.

This she does splendidly. Fifth-century Britain is caught in these pages, and while it may lack some of the exotic glitter of the imaginary 12th-century Britain that Arthur usually inhabits, it is a fascinating place: a land divided between the old Druidic religion and the upstart faith of the Christians, between Germanic invaders and the original Celtic inhabitants, between the remembered order of the departed Romans and the chaotic forces of the advancing Dark Ages.

In the midst of this turmoil, Arthur and his mentor Merlin stand as forces of reason, peace and order, working against considerable odds and wresting at least a temporary victory. If Camelot means anything beyond simple, colorful adventures, this is what it means, and Stewart has conveyed that meaning - not perfectly but as well as faithfulness to her vision will permit. CAPTION: Map, no caption, from "The Last Enchantment"