Fiend

Part of speech: noun (n.)

An implacable or malicious foe; one who is diabolically wicked or cruel; an infernal being; -- applied specifically to the devil or a demon. Into this wild abyss the wary fiend Stood on the brink of Hell and looked a while. Milton. O woman! woman! when to ill thy mind Is bent, all hell contains no fouler fiend. Pope.