One: The old house was silent. Then I heard a child's giggle and saw a shadow that wasn't mine. Two: I woke in the dark. A cold hand touched my face and a voice whispered 'you're not alone' in my ear.
They are effective because they quickly set the mood. In just two sentences, they can introduce a spooky situation like a sudden noise or a strange sight, leaving your mind to fill in the blanks and imagine the worst.