Fear of death. This is the ultimate fear, both existentially and psychologically. ...

The dark. ...

Creepy, crawly things. ...

Scary places. ...

Disfigurement. ...

Dismemberment. ...

Suspense (anticipation and expectations). ...

Spooky music.

The Familiar Made Strange. It's terrifying when something close to the reader or the character starts to look or act differently. ...

Tapping into the Reader's Darkest Fears. ...

Verisimilitude. ...

A Sense of Claustrophobia. ...

A Sense of Paranoia. ...

Violence.