Image: A24

Liminal horror movie Backrooms is set to become A24's biggest hit, with $10.4 million clipping through in preview screenings, earning back its production budget, and signaling a $40-50 million opening weekend. Alex Garland's "Civil War," the company's previous top earner, made $2.9 million in previews.

Adapted by Kane Parsons from his own low-budget, high-concept online series, the film draws from message-board lore an endless maze of faded, yellowing offices, a world inspired by strip malls, business parks, hotel basements and other liminal spaces of suburban America.…