The lines between mystery, horror and adventure are really quite thin. Think about it: there always seems to be a damsel in distress, there’s always a single, malevolent mind dictating the situation, and there’s always a hero of some kind. The main difference in all of them is the occupation of the hero and how he provides a resolution to the problem. In a mystery he’s a detective who solves the case. In an adventure story he’s an explorer (archeologist?) who discovers the truth. In horror he’s an ordinary Joe who glimpses the real truth behind the evil events (and occasionally dies in the process).
The picture above can be any of the three. Pick your poison. It was inspired by the pulp cover below.