Horror has never been Francis Ford Coppola's best genre. There are some nice touches in his '90s version of Dracula, but those generally involved extravagant costume design or his use of early cinema in-camera special effects. The Godfather movies and Apocalypse Now contain their harrowing moments, but few legitimate jump-scares. Jack is probably the closest Coppola came to a true fright-fest but for reasons other than intention or technique.
Dementia 13 is his first fully-credited film and definitely fits the horror movie bill. There's a killer run amok in a Scottish castle. There is occasional blood spilled (i.e., chocolate syrup). It is produced by Roger Corman, king of the chiller quickies. Is it a good horror film? Not really. It's basically a Corman cash-in attempt on Psycho (i.e., don't get too attached to the lead female beyond the first half hour).
I'd heard the story about Coppola getting the job because Corman had some leftover film stock and because the young director promised to pull it off in an insanely truncated amount of time. So I wasn't expecting Citizen Kane. I wasn't even expecting Tetro. But I may have been expecting a little more "Coppola." Other than some nice mood setting scene on a row boat in the beginning and few brief underwater scenes later on, that certain operatic quality simply is not yet there. Perhaps given more shooting time, more film stock and a better script, Francis could have dug in, explored the possibilities. For a first effort, it's not terrible. But other the self-serious line readings of the inimitable Patrick Magee (below), it's hard to go too crazy for Dementia 13.