In James Blish's Black Easter and The Day After Judgment, magic is control of demons,
which are fallen angels, and is practised in the twentieth century
although most people do not believe in it. It is theorized that stable
negative entropy would be eternal life. See here.
In Poul Anderson's Operation Otherworld, set on a parallel Earth, magic, practised in earler centuries, was protogoetics whereas goetics is a new energy source. See here. Demons are inhabitants of Hell, a chaotic, entropic universe.
Thus, Blish and Anderson present alternative imaginative takes on magic, demons and entropy.