Fiendishly

It was fiendishly cold.

His long, claw-like fingers danced from one part of the machine to another fiendishly, and a hideous grin distorted his features.

What a fiendishly clever plan!

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.

It was hot there too; big flies buzzed fiendishly, and did not sting, but stabbed.

The terrible spectacle drove for the instant all thought of rebellion from my mind. My very blood froze in my veins and I stood motionless. I heard Hassim laugh grimly behind me. The eyes in the cadaverous face blazed fiendishly at me and I blanched from the concentrated satanic fury in them.

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.

Balancing horror and comedy is fiendishly difficult. The two rarely work well together, which is why the successes become so lauded (Evil Dead 2, An American Werewolf In London, Dead Alive).