Not sure how familiar you are with the horror genre but hopefully you haven’t seen some of these.

• Martyrs: the original French one, not the remake. Some called it the 2001 of horror movies and I might have to agree with that. Pretty graphic and not an easy watch, but it goes to some interesting places
• In a Glass Cage: from Spain, it’s pretty disturbing and has some good atmosphere and is well made. It stuck with me since I saw it in college.
• Black Christmas: the original. The first real slasher movie (precedes Halloween). Well made with decent writing and some good scares. Probably my favorite ending to a horror movie, getting chills just thinking of it.
• Suspiria: more well know but thought I’d mention it anyway. Both the original and the remake are good in this case. They’re both atmospheric but with wildly different intentions and conjure up different feelings.
• Hausu: batshit insane Japanese horror movie. Just a good time all around.
• Audition: Might be considered more well know but definitely worth checking out.
• Toby Dammit: cool horror short directed by Fellini.
• Woman in the Dunes: more of a psychological movie, but definitely has some horror vibes to it
• Don’t Look Now: also more of a psychological slow burn but an incredible ending and can definitely be considered horror
• The Descent: this one seems to get forgotten about, but worth checking out especially if you are claustrophobic
• Hour of the Wolf: Bergman’s horror movie. Some good atmosphere in this one
• Let’s Scare Jessica to Death: another good atmospheric horror movie
• Sleepaway Camp: notoriously bad but also entertaining
• Session 9: A good psychological horror movie
• Tenebre: another Argento but my favorite giallo of his
• Antichrist: this is a tough watch but beautifully filmed. Also very fucked up and graphic.
• The Strangers: an effective home invasion movie Eden Lake: fucked up but worth a watch. Also has Michael Fassbender.
• Lastly, Society and The People Under the Stairs are fun social commentary horrors