The cessation of existence. Not necessarily scary per se, but most undoubtedly sad, and we only have 800 billion years-ish with which we can reliably hold it off by current estimates.
Technically the universe won't be completely destroyed until black holes die, approximately one googol years from now.
But I'd have to say spiders everywhere.
Like all over you just crawling around falling from the ceiling.
I'm not really absolutely terrified of spiders but that would be terrifying
Follow the path of the spiders.
Anyway, why has no one mentioned the moon in majora's mask. Seriously, that moon seems delighted to crush you.