The story it's based on: "Haunted House"
How scary it actually is: Edging out Sarah just slightly is another ghost from the Bellows house, who we see only twice. The first is the scariest by far, when Chuck, one of Stella's friends, glimpses her through a crack in the door of a bedroom in the old, haunted mansion. Sitting on the edge of an ornate bed and dressed in a black lace gown and veil, she's instantly creepy as hell, and guarded by a massive doberman to boot. When she turns her head towards Chuck, sensing his presence, it's revealed that she's blind and very clearly evil, judging from her twisted expression. (*shudders*)
She proceeds to zoom over towards Chuck in a jump scare, and later nearly gets Stella killed during a creepy haunted house hallucination. Despite knowing essentially nothing about her other than the fact that she was related to Sarah and helped keep her hostage, the movie shows how deep her malevolence is rooted.
Image Source: CBS Films