This is not Asimov's usual style. The book you should be looking for is "Azazel", later re-packaged as "Magic". It's never establishes that Azazel is a demon. T

This is not Asimov's usual style. The book you should be looking for is "Azazel", later re-packaged as "Magic". It's never establishes that Azazel is a demon. The book vacillates between "demon" and "technologically advanced extraterrestrial being".