In Japan, Fries licensed the film to TCC/GAGA Communications which more accurately marketed it as an action flick but kept the Demonstone (デモンストーン) title. That's exactly what it is, aside from a hammy supernatural twist, and rests comfortably in the slew of cheap action flicks shot in the Philippines in the late '80s. This could have easily been entitled something like "Quest for the Devil Stone" with no indication of the slight horror/sci-fi angle and no one would have been the wiser. But since this was purported to be a horror movie on VHS, it's not hard to imagine many a disappointed rentals.
Although this all takes a backseat to chessy one-liners ("you already took my family jewels once so this time you'll have to take a chunk outta my ass!"), actors mysteriously dubbing over themselves in random scenes, and a horrifically bad synth score. Mostly, Demonstone is an excuse for Vincent and Ermey to shoot/blow up piles of Filipinos and engage in chases with speeding junkers and armed dirtbikers in a flick that could have just as easily been helmed by Bruno Mattei somewhere in-between his Cop Game (1988) and Born to Fight (1989). So if you like this very specific niche of action movie, this serves as an interesting take that you might have otherwise passed on due to shady marketing.