A half-century after the original helped rewrite the rules of horror and launched a thousand imitators, "The Exorcist: Believer" tries picking up that mantle, with the lure of 90-year-old Ellen Burstyn reprising her role. The nostalgia factor elevates an otherwise slow-building film that maintains an eerie creepiness before fumbling through a slightly muddled climactic act.

It's hard to overstate how shocking the original "The Exorcist" was in 1973, based on William Peter Blatty's novel about demonic possession, changing, among other things, how a lot of people look at pea soup. A trio of widely spaced sequels, one prequel, endless cable specials and countless knockoffs have followed, further evidence that when a title is this iconic, it's hard to keep a good demon down.

That seems to be the plan with "Believer," which hails from horror factory Blumhouse and director David Gordon Green, who oversaw the recent "Halloween" trilogy. Yet franchising this property presents a formidable challenge, since "The Exorcist" derived its fear factor from an understated dread every time someone approached the stairs to the possessed girl's room, unlike the jump-at-you scares and ostentatious gore that have become the status quo in the intervening decades.

"Believer" essentially tries to wed those two slightly disparate impules, which leaves the film in a sort of in-between land in terms of expectations of modern horror and honoring its roots. The same goes for Burstyn's extended cameo, as the frantic dad, Victor (Leslie Odom, Jr.), grudgingly reaches out to her Chris MacNeil, who it turns out wrote a book detailing her ordeal.

Victor is raising 13-year-old Angela (Lydia Jewett) alone, having lost her mother when the girl was born. The two seem happy enough, which doesn't prevent Angela from venturing into the woods with her friend Katherine (Olivia O'Neill), engaging in a ritual that might help her contact her late mom.

The girls disappear for a time, and everyone's relieved when they turn up -- at least, before their behavior becomes increasingly odd, then disturbing.

The film's at its best during that stretch, as Katherine's parents (Jennifer Nettles and Norbert Leo Butz) turn to their church for guidance, while Victor fields unsolicited advice from his concerned neighbor ("The Handmaid Tale's" Ann Dowd). Jewett and O'Neill also do yeoman work in flashing glimpses of evil, making them even scarier than the average teenager.

Frankly, if the movie stayed within that key it would be more effective, even if that meant sacrificing hard-core horror fans. The finishing kick, though, tries to segue into more conventional horror and gets lost in the pyrotechnics, providing a stark demonstration of the adage "Less is more."

Between the Burstyn boost, small touches like incorporating the spine-tingling original music "Tubular Bells" and the uneasy first half, "The Exorcist: Believer" still proves a moderately worthy heir, though the intention to develop this into another trilogy, a la "Halloween," leaves ample skepticism about whether there's enough meat on the bone to slice away at it any further.

The most comforting aspect of "The Exorcist," after all, was the prospect -- in the quiet wake of all that pain and sacrifice -- of an amnesiac Regan and her mom getting on with their lives. While "Believer" ultimately hangs on the notion of faith, the only thing worse than having too little of it, potentially, is having too much of it.

"The Exorcist: Believer" premieres October 6 in US theaters. It's rated R.