The movie provides a wormhole. And hellspawn. According to the film's continuity (based loosely on all three games), a gene exist that indicates a person's predisposition toward good or evil. Once infected by whatever is on the other side of that wormhole, humans mutate into demonic representations of the darkness in their souls. Contrived? Sure - but it beats the hell out of a demonic invasion force that hides in closets.
The upshot is that all the demons you've known and fragged - Imps, Pinky, The Hellknight - will make their way into movies theaters this October.
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