That is a tough question! I think it depends on what you define as horror? I have trouble here because I don't like horror, but if something is horror-y and does other stuff I like, the fact that it's also horror doesn't turn me away, and I may fail to notice it's horror? And especially with SF/fantasy it can be sort of ambiguous anyway. I describe them as Gothic novels (that are also goth) because they do fit the Gothic genre most comfortably I think, and gothic fiction is also often in that liminal place between horror and not.
The basic plot of the first book, stripped of the aesthetic and interstellar necromancy, is "group of people trapped together in creepy mansion, one of them is a killer and they have to figure out who it is before they kill again." That can be horror but it can also be, like, Miss Marple? I came to it like "Oooh, trope-y country-house mystery!" but you could probably also read it as horror and in terms of tone it's probably halfway in the middle, it's a little too intense for a cozy but probably not all the way to slasher movie.
The second one is probably closer to outright horror but it's much more psychological horror - the POV character spends the whole book unsure at any point if what she's perceiving is real or her existing mental illness or a new trauma reaction or gaslighting or supernatural weirdness, and so does the reader, and that can get pretty intense.
Also there is a lot of gore. Not like slasher movie gore, but the necromancy isn't a few aesthetic zombies or glowy spirits, it's "we deal with dismembered human body parts and deformed disembodied organs as part of our day-to-day work, you want to help render some human fat into soap today? It's laundry tomorrow." That's something I handle fine in text but probably couldn't in visuals; ymmv.
(It also is written in a very distinctive style that is either love-it or hate-it - I think you just have to try it and see if it works for you in that way.)
General plot spoilers for Locked Tomb
The basic plot of the first book, stripped of the aesthetic and interstellar necromancy, is "group of people trapped together in creepy mansion, one of them is a killer and they have to figure out who it is before they kill again." That can be horror but it can also be, like, Miss Marple? I came to it like "Oooh, trope-y country-house mystery!" but you could probably also read it as horror and in terms of tone it's probably halfway in the middle, it's a little too intense for a cozy but probably not all the way to slasher movie.
The second one is probably closer to outright horror but it's much more psychological horror - the POV character spends the whole book unsure at any point if what she's perceiving is real or her existing mental illness or a new trauma reaction or gaslighting or supernatural weirdness, and so does the reader, and that can get pretty intense.
Also there is a lot of gore. Not like slasher movie gore, but the necromancy isn't a few aesthetic zombies or glowy spirits, it's "we deal with dismembered human body parts and deformed disembodied organs as part of our day-to-day work, you want to help render some human fat into soap today? It's laundry tomorrow." That's something I handle fine in text but probably couldn't in visuals; ymmv.
(It also is written in a very distinctive style that is either love-it or hate-it - I think you just have to try it and see if it works for you in that way.)