fiendishly

In a fiendish manner; evilly, wickedly.

At a side door, reclined on a couch, two guards of the haram, with their naked swords grasped in their hands, and features, fiendishly contorted between sleep and dissolution, seemed to menace death to any who should venture to approach.

[…] I was suddenly awakened by a savage looking fellow standing close to my bedside, ordering me and my companion to leave our bed forthwith or suffer the consequences; at the same time using the most blasphemous language, and uttering the most hideous oaths imaginable, while fiendishly striking his huge fists in close proximity to my face.

Extremely, very in harsh or negative contexts.

Either the Russians are so fiendishly clever that they've managed to keep one of their agents running in this organization for over twenty years or they're so fiendishly clever that they're sending us false defectors to claim that they have.

In this operation, veins or arteries are taken from various body parts and used to bypass blockages or narrowings in the coronary arteries, those fine, fiddly, yet fiendishly important vital suppliers to the heart muscle itself.