fiend

A devil or demon; a malignant or diabolical being; an evil spirit.

what God or Feend, or ſpirit of the earth, Or Monſter turned to a manly ſhape, Or of what mould or mettel he be made, […]

Be that word our sign of parting, bird or fiend!

A very evil person.

An enemy; a foe.

We waited for our fiend to arrive.

The enemy of mankind, specifically, the Devil; Satan.

He proffered a pact to Satan, calling upon the Fiend and working himself into a frenzy - but his infernal majesty failed to respond.

At the confirmation ceremony the bishop would lay his hands on the child and tie around its forehead a linen band […]. This was believed to strengthen him against the assaults of the fiend […]

An addict or fanatic.

dope fiend

He's been a jazz fiend since his teenage years.

To yearn; to be desperate.

I play it off, but I'm dreaming of you / And I'll try to keep my cool, but I'm fiendin'

I am back in San Francisco at the Clift Hotel, fiending for my fix.