The scene was horrifying. Here, ‘horrifying’ is a/an-

Participle used as adjectives.Present participle adjectives amusing, boring, tiring etc., are active and mean ‘having this effect’. Past participle adjectives amused, horrified, tired etc,. are passive and ‘affected in this way’.The scene was horrifying. (The spectators were horrified.)Source: A practical English Grammar, Oxford University Press.