Vampire & Victim costume
Here's a costume for a couple going to a party.
Vampires are huge nowadays, so this involves the guy dressing up as a modern day vampire...meaning regular clothes, but a pale face, and of course, fangs. Blood on the chin would also help. The girl is the victim, which would involve her as having a somewhat pale face and teeth marks on her neck. Again, blood around the wound and maybe even on her shirt collar would bring a greater effect.
This would also work ViceVersa, where the girl is the vampire and the guy is the victim.