At the risk of repeating myself, I still get a little excited when I find a horror movie from the eighties or nineties I haven't heard of. I really should know better by now... I've been seeking them out for decades now, and chances are there's probably a perfectly good reason why these obscure movies remain obscure.
Deadly Manor is very much an object lesson about this: the only things of note here are that it's written and directed by José Ramón Larraz (the final English-language movie from the creator of the iconic '70s Eurosmut film Vampyres), and that it's a particularly inept example of the slasher genre.
A bunch of attractive young adults on their way to a holiday elsewhere decide to spend the night in a spooky abandoned manor in the woods. There's some weird shit in the house (a shrine to a wrecked car, pictures of the same woman in various states of undress all over the place, signs that the house might not be uninhabited and, uh, a couple coffins in the basement) which the kids proceed to comment on and subsequently ignore. Then they start getting killed by a disfigured woman and her weirdly-accented beau.
Yes, it's a literal Scooby-Doo plot, except that calling it that does Hanna Barbera, even at their laziest, corner-cutting worst, a disservice. They at least knew their way around a plot structure.
Every single aspect of this movie is botched, sometimes hilariously so. The cinematography is basic, the effects laughable (the only one worth commenting on is the villain's makeup, which is... decent). The acting is actually the film's biggest asset, and not in a good way; The combination of the actors' community theatre (or porn) -level skills and the script's vapidness make their line readings consistently funny.
The script doesn't work at all, either: None of the characters ever act like a human being (this is one of those movies that seems like it was written by aliens who've only studied us through bad movies), the mystery and its explanation is inane, most plot developments feel like padding. There's at least an attempt to build up some atmosphere, but no one involved is even remotely up to the task.
More importantly, the film is also completely incompetent in providing slasher thrills; All the kills are basic stabbings and throat slashes. There's no imagination to the deaths or chases, no tension building, and no possible attachment (positive or negative) towards any of the victims, so the murders are weirdly inconsequent - it just feels like the film is going through a checklist. The best kill, for all the wrong reasons, is just another throat slash, but filmed in an overlong slow-motion shot.
It all leads up to a literal cop-out (emphasis on cop) of an ending, a wildly terrible capstone for a movie that had no reason to exist except that the producers found a neat, cheap derelict house they could shoot in.
The only remaining question at this point is: is it so bad it's good? And the answer is a firm "kind of". The acting made me laugh far more than it should have, but there's no escaping the fact that Deadly Manor is utterly mediocre, soulless, and full of padding. I wouldn't recommend going into this one alone, but armed with like-minded, mean-spirited compatriots (thank you, Moviesign!) it can be a relative good time.