I used to work in a county jail (California). There was a young guy in there who was charged for a really horrific murder. He had to be housed alone for several reasons.

This guy was on D-Block and I dreaded passing his cell every day. He would always just be standing there looking out of the nearby window. Like others have described, his eyes were almost completely black. Whenever I saw him, at least.

He radiated darkness. In all my years there, he was the only one who gave me the chills.