You are really drawing me in with the Magnus Archives! I'm not sure about horror but I'm very tempted...
In case it's of any use -- my post here has quotes from various people on the subject of how "horror-y" it is (which is obviously an impossible question to answer, but I think the quotes give some idea of what kind of thing it is and what it isn't, so people can gauge whether it might be their idea of a good time or not):
All non-spoilery! (Apart from things like "there's a plot" and "there are characters other than Jon", which you will already have gathered from this post. *g*)
no subject
In case it's of any use -- my post here has quotes from various people on the subject of how "horror-y" it is (which is obviously an impossible question to answer, but I think the quotes give some idea of what kind of thing it is and what it isn't, so people can gauge whether it might be their idea of a good time or not):
https://rydra-wong.dreamwidth.org/732843.html
There's also good discussion here:
https://findingfeather.tumblr.com/post/189766118082/on-the-one-hand-i-do-get-what-this-post
And my personal rec post about why I love the show is here, FWIW: https://rydra-wong.dreamwidth.org/735506.html
All non-spoilery! (Apart from things like "there's a plot" and "there are characters other than Jon", which you will already have gathered from this post. *g*)