We've all probably played a custom horror map at some point. Many of them, however, consist of little atmosphere or buildup and rely only on cheap jump scares (like "screamers") and ridiculous amounts of blood and gore.
A map that keeps the player on his toes all the time, however, one with much atmosphere and few (if any) jump scares, is far scarier. A map that, for instance, uses the unknown, or outright bizarre and surreal, to disturb the player without using a bogeyman that jumps from around a corner, without even a single drop of blood.
So now I ask you: what makes a horror map truly scary?