Demonic (adjective)

1. Of, relating to, or characteristic of demons.
2. Having a cruel and evil nature; fiendish.
3. Reminiscent of a demon, especially in being fiercely intense or active.

Origin:

From late latin dæmonicus, from latin dæmon ("demo.n")

Examples:

1. The crowd jeered and catcalled, their behavior becoming increasingly demonic.
2. The demon's voice was low and demonic, sending shivers down her spine.
3. The painting had a demonic expression on the face of the subject.
4. He had a demonic grin on his face as he plotted his next move.
5. The storm had a demonic energy, battering the coast with relentless force.