It sounds promising enough, at first. Exactly 12 years after the events of "The Exorcist," the Washington area is witness to a series of grisly occult murders, and the killer or killers may well be supernatural. Characters from the earlier novel are back in action, confronting the forces of darkness. The first victim is a little boy who delivers The Washington Post; he's found crucified to a pair of oars in the Georgetown University boathouse. Then a couple of local priests are polished off in an equally gruesome manner. It sounds as if we're in store for another horror blockbuster.

No such luck--for this time around Blatty has made a peculiar decision: he's chosen to tell his story from the point of view of a garrulous old police detective, Lt. William Kinderman, a veteran of 43 years on the force. Kinderman played a supporting role in "The Exorcist," but now, in a kind of fictive Peter Principle, he's been elevated to the position of central character. And a more tedious character could scarcely be imagined.

From the start, he displays little interest in the crimes he's supposed to be investigating; he's far more concerned with such theological problems as the existence of God, the problem of evil and the nature of the human soul. As a result, until the novel's final chapters, the events of the story take a back seat to Kinderman's interminable soliloquies on the Bible, quantum mechanics, electrons, the Neanderthal, elephants, chimpanzees, Shakespeare, Occam, Blake, Leopold and Loeb, genetic theory, quarks, neutrinos, Kafka, Teilhard de Chardin, B. F. Skinner, Darwin, Alfred Binet and the role of the Prime Mover. It's typical of the book that one of the first clues in the case--that two strands of hair match--is revealed only after we've been forced to sit through a meandering eight-page discussion of the Gnostics, angels and the miracle of the autonomic nervous system.

That Blatty would rather write a novel of ideas than a horror novel seems a perfectly laudable ambition; "The Exorcist," in fact, managed to be both. But "Legion" is long-winded, intrusive and, in light of the book's simple-minded supernatural ending, ultimately pointless.

There is a plot, of sorts, hidden somewhere amid the philosophizing: It's discovered that, though each of the murders is different, certain aspects of the modus operandi, including the severing of the victims' index fingers, ominously parallel that of a San Francisco psychopath called "the Gemini killer," who was believed shot to death back in 1971. And while Kinderman is busy grappling with the big questions, dimly in the background there are bodies piling up.

But you have to keep a sharp eye out for them, for speculations like this keep getting in the way: " 'What we see is only part of the spectrum,' brooded Kinderman, 'a tiny slot between the gamma rays and radio waves, a little fraction of the light that there is . . . So when God said, "Let there be light," he pondered, 'it could be that He was really saying,' "Let there be reality." ' " In the face of such brooding and pondering, his partner Atkins, we learn, "didn't know what to say."

Poor Atkins, in fact, is not a character at all, he's merely a foil for Kinderman, patiently listening to his monologues. When Atkins tries to interject a thought of his own, the old windbag interrupts with, "Wait, I'm not finished." Even in the final scene, with Kinderman sitting in a White Castle still yakking--"You know, we talk about evil in this world and where it comes from. But how do we explain all the good?"--Atkins is made to say: "Would you do me a favor, Lieutenant? Would you please explain your theory?" Kinderman is, of course, only too glad to oblige, and as the novel ends there's no sign he'll ever shut up: "The physicists now are all certain that all the known processes in nature were once part of a single, unified force . . ."

If Atkins is cardboard, other characters are trundled onstage merely to be butchered. A nurse we've barely met gets hers in a typically grisly little throwaway: "Her torso had been slit open, her organs removed, and her body--before being sewn back up--had been stuffed with light switches taken from a storeroom in the hospital basement." Clearly from the way they're presented, such deaths are intended to titillate.

Suddenly, in the book's closing pages, Blatty becomes all business, staging several confrontations between Kinderman and the killer during which, in another artificial device reminiscent of old detective films, the latter simply looks Kinderman in the eye and, in a brief lecture delivered with cool villainly hauteur, Explains All. "Haven't you guessed it, Lieutenant? Why, of course you have. You've finally put it all together." There's also a mildly dramatic scene in which Kinderman realizes his own family is in danger and ends up dashing home to save them. It's as if the author realized, just before the end, that he has a responsibility to tell a story. Kinderman moves quickly for an old man; but for Blatty, it's much too late.