'헉' (or '헐') is an expression many young Koreans use to show they are shocked about something.
It is similar to 'Oh my god' in English.
But often '헐' is considered as casual expression not formal at all.
So old people would not never use that expression unless they are trying to be humorous.
Ex) 헐! 저기 차 사고 났어! = Omg, car accident just happened over there.
헉! 뱀파이어? 너가? = Omg, a vampire? Are you?
And formal way of saying '헉' or '헐' can be '세상에.'
Old people may use that expression when they get shocked.
Hope it helps!