Vampiric

Zak and Delia are still trying to figure out how to transition from friends with benefits back to just friends and haven't hung out in a while. At a party they find a BDSM magazine and call a disturbing personal ad as a joke.

The call brings Dog, a vampire who loves pain and forces Zak and Delia to torture her to death, or so it seems. But Dog comes back, again and again, leaving murder and monsters in her wake. Her victims become twisted ghouls, hungry for human flesh.

Delia, a lapsed Catholic, realizes Dog's true nature and that turning to religion may be their only way of fighting back. Atheist Zak is forced to a similar conclusion, but goes a different way.

In the final confrontation Delia and Zak plan to stay with Dog until sunrise, satisfying her desires so she doesn't hurt anyone else. That goes south immediately when Dog kills Zak and drains him of blood. She reveals her sexual obsession with Delia and her plan to make Delia her companion in eternity.

Delia finds a way to kill Dog suddenly, without the torture she now realizes is part of a ceremony Dog needs to maintain her immortality. But she knows Dog will still come back, enraged this time. Zak reappears, revealing a deal he made with a demonic spirit that now inhabits his body, and prevents Delia's escape. They have to finish this.

Dog returns to her body, not as strong as before but still deadly. Zak doesn't help in the fight. He just tells Delia the religious invocation that opens Hell and Dog finally faces her judgement.

It's over. Zak seems to die and come back again, but is it really him in his body? That's the question in the deliberately ambiguous ending.