Unknown Assailant

The Unknown Assailant is a staple of crime fiction, but also appears as a plot device in every other kind of fiction imaginable, dating back hundreds of years. There are way too many examples to list (although I'm sure everyone will add some anyhow). The assault may be mental or spiritual instead of physical.

The key to the device is the dramatic tension created by the mystery of the assailant's identity. Cases where the assailant remains unknown and no one cares, such as being clubbed by a stranger in a brawl, are not an instance of this trope.

Anime and Manga

 * The masked assailant in Under Grand Hotel remains a mystery for a long time until they're seen hiding the mask.

Comic Books

 * In the first Sin City story, Kevin the Serial Killer was unknown for much of the story.