Nergal is a god worshiped in Babylon in SM Stirling's Against The Tide Of Time and a demon in DC horror comics. See image and here. John Milton's Paradise Lost explains polytheism by stating that, when demons came to Earth, they not only tempted mankind but also masqueraded as all the pagan gods so that they would be, inappropriately, worshiped. Thus, Nergal as a Babylonian god and as a contemporary demon could be two parts of a single story. (Greeks explained Egyptian zoomorphic deities by stating that, when Titans attacked Olympus, the gods fled to Egypt and hid in animal forms.)
Poul Anderson wrote fantasies about Norse gods but not much about demons. The Devil's Game has a literal Devil or a powerful Devil-like being or a mental condition of one of its human characters. Anderson wrote a humorous short story about a scientist tricking a literal demon but I remember neither the title nor the plot. In Poul and Karen Anderson's The King Of Ys, Corentinus/St Corentin, the Christian minister in Ys, regards the Gods of Ys not as delusions but as demons, thus agreeing with Milton. If these Gods did exist, then some of their actions could indeed be described as demonic. The Sea God, Lir, is feared and appeased, not loved or thanked, and winds up destroying the city. In the Biblical narrative, the sea is the primordial chaos driven back by God at the creation.
The Ysan Gods, Lir in particular, have a similar unsettling feel to the demons in James Blish's Black Easter, although there are three differences:
the Ysan deities threaten the city, not the world;
they remain off-stage;
they are a Triad whereas Blish's demons number seven million, four hundred and fifty thousand, nine hundred and twenty-six.
The Following Morning: This post was not meant to be exhaustive but to seek out odd details like Nergal's dual nature and Blish's precise number of demons. However, for completeness, we should mention the demonic hordes in Anderson's Operation Chaos.