I'm in a desperate search for a horror/mystery something movie...

If you like slashers, check out Behind the Mask: The Rise of Leslie Vernon.
If you like disturbing, Funny Games
If you like actually terrible movies with one redeeming quality, check out Terrifier (Art the Clown is the ONLY reason to watch it)
The Poughkeepsie Tapes is pretty interesting too.

2018's movie Winchester
1997's movie The Edge

Hereditary
Midsommar
Annihilation
The Ritual

"Dismissed" was surprisingly good.

Knives Out is mystery, I really enjoyed it!

Silent Hill: Revelation
Stepfather 1 & 2

The Dare, brutal

2018’s Searching was a good mystery

If you like true story stuff check out the Devil Next Door Series: Pazuzu Algard