10 Horror Movie Mysteries That Will Never Leave You

In Black Christmas, sorority sister Jess Bradford (Margot Kidder) is constantly called by a person who's only referred to as Billy. Each time Billy calls the house, he murders one of Jess' friends soon after.

To Jess' horror, she eventually discovers the phone calls are coming from her house, meaning Billy has been hiding there the entire time he's tormented her.

Because Black Christmas is a slasher, first-time watchers naturally assume the film will culminate with a big twist when the murderer is revealed. And when Jess kills her boyfriend Peter under the impression he's the caller, it looks like everything has wrapped up. But in the closing moments, Billy's voice is heard from the attic, proving he's still alive.

Not only is this a chilling ending, it's perplexing, since Billy is never identified. Apart from a single shot of Billy's eye, his face is never seen. It's never explained where he came from, why he lives in Jess's house, or why he targeted her. Even though Black Christmas came out nearly 50 years ago, we're still no closer to getting an answer.