Most terrifying villains that we never see?

Or don’t see for a large part of a story and by the time we do they are already terrifying from what we know of them.

I was thinking Cameron’s dad from Ferris Bueller. We never see even a picture of the guy but his threat is so terrifying, almost embodying the threat of abusive parents. What an achievement in writing to make us fear for Cameron so much without even meeting his father.