I'm having a brain block!

<p>Okay, so I'm trying to think of the name of a certain literary device or term or whatever but I can't think of it so I'm hoping that someone here can tell me. What I'm thinking of is when the author of a horror movie or book doesn't show you or tell you what the killer/monster looks like, so your mind creates the scariest thing imaginable to yourself. </p>

<p>Does anyone know what its called?</p>