Demons and serial killers.
I don't find demons to be particularly scary. This started out pretty much as a retelling of The Exorcist, actually referring to it more than once. So I was prepared to be totally unimpressed.
However, it takes a sharp turn into 'holy shit that was unexpected' after the rites are concluded.
The serial killer part skirted heavily on the edge of unbelievable - how did he think he would get away with it for much longer? but was attention holding. However, I suppose that something serial killers don't really consider.
And what does happen to demons that have been cast out of their human hosts?