Lucretius invokes Venus before arguing that the gods do not intervene. The Lusiads describes Portuguese explorers uttering Catholic prayers but, because this is an epic poem, being answered by Olympians. (I am coming to Poul Anderson but want to locate my remarks in this wider context.)
I say this because theological and literary accounts of "demons" can part company. Theologically, each demon is a pure spirit and a fallen angel. Thus, CS Lewis' Screwtape cannot possibly have a nephew who is a younger demon undergoing training. Were such younger demons born in Hell instead of falling from Heaven? Does Screwtape have a brother or sister who was a parent of Wormwood? You might reply that this aspect of the fictional correspondence is not meant to be taken seriously. I appreciate Lewis' moral insights but dislike his frivolous treatment of the serious concept of damnation. Mike Carey's angels and demons have family relationships that do not fit with pure spirituality.
The "demons" in Anderson's Operation Chaos are simply not demons. They are physical organisms inhabiting the material environment of a planetary surface in another universe. In past ages, they interacted with Terrestrial human beings and were able to play tricks that made them seem to be supernatural. This "Hell" is a place that can be militarily invaded from Earth. It is not an evil hereafter for human beings. It contains one demonic lord who answers the description of Hitler - although the World War II of the goetic timeline was fought against the Caliphate, not against the Axis. Has the Hitler from our timeline or a Hitler who never came to power in the Germany of the goetic timeline traveled to "Hell" and risen to power there or, alternatively, has a demon assumed Hitler's appearance, mannerisms and language for some reason? In either case, human souls from the goetic Earth may be damned in the sense of being separated from the Highest but there is no evidence that they then migrate to what is described as "...the hell universe." (p. 256)