expression, godan verb (-su)
to bluff; to make empty threats; to intimidate by appearance
Idiomatic expression meaning to try to scare someone with a fierce look or empty threats, like a demon mask. Often used when someone's bark is worse than their bite.
彼は鬼面人を驚かすようなことを言うが、本当は優しい。
He says things that are like empty threats, but he's actually kind.
そんなに怒鳴っても、鬼面人を驚かすだけだよ。
Even if you yell like that, it's just bluffing.