noun which may take the genitive case particle 'no'
scariest; most frightening
Slangy, emphatic way to say something is the scariest or most terrifying. Often used in casual conversation, online posts, or entertainment contexts.
この映画、マジで最恐だった。
This movie was seriously the scariest.
あのホラーゲームは最恐の体験だよ。
That horror game is the most frightening experience.
Slang coinage from 最 (most) + 恐 (fear/scary), likely a playful twist on the homophone 最強 (strongest).