na-adjective
Describes a fierce, brutal, and merciless nature, often used for people, actions, or expressions. Somewhat literary and emphatic.
彼の獰悪な表情に、誰もが震え上がった。
Everyone trembled at his fiendish expression.
その犯罪はあまりにも獰悪で、言葉を失った。
The crime was so brutal that I was left speechless.