Decent watch at best, probably won't watch again, and can't recommend unless you just love classic gore practical effects.
I'm honestly confused why this was such a bit hit at the time. Maybe it was just part of the trends, maybe the 1980s was just when people wanted to see horrific gore, and there was a lack of options.
Don't get me wrong, I don't think it's bad. I don't feel like we get a proper explanation of what the hell is happening, so no expectations is really built to be subverted, but I certainly wouldn't want this movie to explain anymore than it did.
I'm honestly not sure exactly what to say about it: it's a "don't mess with dark forces you can't possibly understand" trope with a bunch of blood, gore, and weirdly designed possession.
Part of what I enjoy about (even paranormal) survival situations like this is the idea that their is / was a way out of it, if the characters can figure it out. This is more of just someone blew up a dam, and now we're going to die from a flood.
I just feel like this movie could have been a lot more, and I'm not going to remember this, but I had a good enough time watching it.