Horrifyingly is the adverb: it says that something happens, or is true, in a horrifying way — extremely badly, and in a way that frightens or upsets people. You can put it at the front of a sentence to show the whole fact is a shock: Horrifyingly, nobody had checked the door. You can also put it before an adjective: the scores were horrifyingly low. Though actually, like horrifying, people stretch it to mean “to a shocking degree” even when fear is not the main feeling — a horrifyingly high bill, a horrifyingly long wait. It still sounds strong, not playful.