We can imagine greater horrors than anything most movie makers can conjure up visually. It’s the unseen unknown that truly terrifies.

And for about 25 minutes, “The Void” lives up to that credo and the promise of its title. We witness a killing spree. A sheriff’s deputy collects a bloodied survivor and gets him to a tiny remote hospital.

Almost instantly, the violence explodes inside the building, and the hospital itself is under siege, encircled by mysterious hooded figures with giant black triangles on their cowls.

What’s going on here? We’ve barely caught our breath long enough to figure out the deputy’s (Aaron Poole) marginally competent and over-matched, the staff (Kenneth Welsh, Kathleen Munroe, et al) is isolated and in shock. Leave one of them alone, and she just might kill a patient.

A nurse trainee (Ellen Wong) is the last person you’d want to lean on in a crisis.

“Statistically, you’re much more likely to die in a hospital!”

There’s a pregnant girl (Grace Munro), her grandpa (James Millington), a state trooper (Art Hindle), and the fellow leading that killing spree (Daniel Fathers) has arrived to finish the job, not-exactly-explaining what the escaped victim (Evan Stern) did to bring all this horror down on them all.

“Yeah? What about you, Man of Mystery?”

“Mind your own business!”

And then, wham! We see tentacles crawling out of the mouths of the dead. We see the giant blob these belong to. The mystery evaporates and the siege and the movie built around fritter away every iota of tension that a good-at-showing-terror cast and the scenario have built up.

“The Void” devolves into a creature feature. The legions of hooded cultists, the people inside who “turn,” all routine and gory and not the least bit frightening.

Killer cult pictures are almost as common as monsters among us films. But the moment co-writers/directors Jeremy Gillespie and Steven Kostanski turn their movie over to the blob wrangler, “The Void” — with its wise-cracks and crackling, static-filled radios, it’s desperate dashes to fetch a shotgun from a police cruiser — exhausts all interest and falls apart.

The blob isn’t nearly as scary as something we cannot see. Nothing they do to finesse that — human acolytes of the blob, cult leaders — fixes that hole in the middle of “The Void.”

MPAA Rating: unrated, with graphic violence

Cast: Aaron Poole, Ellen Wong, Kenneth Welsh, Kathleen Munroe, Grace Munro, Daniel Fathers

Credits: Written and directed by Jeremy Gillespie, Steven Kostanski. A Screen Media release.

Running time: 1:30