utter villain; atrocious fiend; devil
Strongly negative term for a person of extreme evil; often used in dramatic or literary contexts.
He went down in history as an utter villain.
The atrocious fiend in that story evokes strong disgust in readers.
悪人 is a general term for a bad person or villain, while 極重悪人 intensifies it to an extreme degree of evil.
極悪人 also means a heinous villain, but 極重悪人 adds 重 (heavy) to emphasize the weight or severity of the evil.
Compound of 極 (extreme), 重 (heavy/severe), and 悪人 (villain). The exact historical derivation is uncertain, but the combination intensifies the notion of a deeply evil person.