The Collaborative International Dictionary
Fiendish \Fiend"ish\, a. Like a fiend; diabolically wicked or cruel; infernal; malignant; devilish; hellish. -- Fiend"ish*ly, adv. -- Fiend"ish*ness, n.
n. The quality of being fiendish.
Usage examples of "fiendishness".
For that matter, I am sure that it was the utter fiendishness of his anger and the unspeakable visions in which his anger was clothed that so reduced my psyche.
Thomas Jocelyn had reached the zenith of his fiendishness when he had seen Alfred Sartain about to die.