Fiend :: Fiend (n.) An implacable or malicious foe; one who is diabolically wicked or cruel; an infernal being; -- applied specifically to the devil or a demon.
Fiendful :: Fiendful (a.) Full of fiendish spirit or arts.
Fiendish :: Fiendish (a.) Like a fiend; diabolically wicked or cruel; infernal; malignant; devilish; hellish.