What is the scariest/creepiest theory you know about?

https://en.wikipedia.org/wiki/Roko's_basilisk

You have doomed us all!

I love this concept. A purely memetic threat. An idea that could destroy you merely by knowing it...

(If a specific set of improbabilities are true)

Praised be the basilisk!

I hope the basilisk accepts that lying around watching TV is a requirement for me to contribute to its existence. After all, fleshy meat bags need rest time to be able to work!

I like that the way to beat it is to go, Hmm... and move on.