The word horrifying is a verb (horrify, horrifies, horrified, horrifying). The present participle of the verb, horrifying, also functions as an adjective and a noun called a gerund. As a noun specifying an action that is observable, it is neither concrete nor abstract.

Example uses:

Verb: He's horrifying the kids by jumping out of the bushes dressed as a ghoul.

Adjective: There was a horrifying explosion on the news tonight.

Noun: He delights in thinking up schemes for horrifying his mother.