He's a 'fiend' for physical fitness. [=he is very devoted to physical fitness]
She's been working 'like a fiend'. [=working very hard]
a dope/drug 'fiend' [='addict']
(..) ten thousand men. Our power is ready. Our lack is nothing but our leave. Front to front you bring thou this fiend of Scotland and myself, and within my sword's length you set him. If he scape, heaven forgive him too. LENNOX: 'Great Dunsinane he strongly fortifies. (..)2015 Macbeth