I've been revisiting some Edgar Allan Poe stories I haven't read in a while, which has got me thinking about Freud's theory of the death drive. And somehow that has filtered into thinking about Krevborna's demons and devils.
One of the grand things about making your own campaign setting for D&D is that you can choose to exclude as much of the inherited D&Disms into your world as you want or bend those D&Disms in a way that wouldn't necessarily work in an already established setting; you can pick and choose from the "canon," or discard it entirely.
Since Krevborna was my first "5e setting," I wanted to play around in D&D's sandbox without feeling beholden to the way D&D tends to do things. For the purposes of illustrating what I'm getting at here, I'm going to talk about what differentiates demons and devils in Krevborna.(1)
According to the 5e Monster Manual, demons are "the embodiment of chaos and evil--engines of destruction barely contained in monstrous form" and devils "live to conquer, enslave, and oppress." Looking at those two basic building blocks of differentiation reminded me of Freud's theory of the death drive. According to Freud, the instinct toward death, destruction, and dissolution can take a myriad of instinctual forms. As he states in "The Economic Problem of Masochism," "The instinct is then called the destructive instinct, the instinct for mastery, or the will to power." Demons, then, with their "embodiment of chaos" and function as "engines of destruction," are clearly manifestations of the death drive as a pure destructive instinct.(2) Devils, on the other hand, as beings who "live to conquer, enslave, and oppress," are manifestations of the death drive as the instinct for mastery and the will to power writ monstrously large.(3)
Sure, that's a neat theoretical congruence, but what does it mean in terms of world building? First, it helps explain the "Blood War" between demons and devils that was a big part of 2e's canon. As manifestations of a cosmic death drive, you might expect that devils and demons should be allies against the life-loving races of the world, but although they are manifestations of the same force each group is so focused on their particular flavor of the death drive that it excludes the methods and schemes of the other. The will to power of the tyrannical devils is simply incompatible with the demons' chaotic urge for obliteration, and vice versa. A libido divided against itself cannot stand.
Second, this strife between two supernatural forces out to either enslave or destroy mankind gives mankind a profound weapon against both: as more and more of the nature of these beings is revealed in play, it gives the characters a natural tactic that can aid them in the struggle against cosmic darkness: either side could be carefully leveraged against the other to keep both of these death instincts in check--a kind of libidinal stalemate. Even if the total defeat of demons and devils is impossible for mere mortal agents--and can the death drive ever truly be banished entirely?--the way in which they can be pitted against each other keeps things in necessary stasis. It isn't so much that the Moorcockian struggle between law and chaos is a substitute for good and evil, it's the notion that destruction and tyranny must be maneuvered into a stable state for the greater good of the continued existence of all.(4)
(1) - The general populace in Krevborna would recognize no practical difference or theological distinction between demons and devils, of course. Superstition and inherited belief masks the real cosmological truths that govern the universe.
(2) - Since Orcus is a a demon lord associated with the undead, this also colors the setting's view of what undeath is: it isn't an orderly process, it's life inverted into deathless chaos and always already an impulse toward decay.
A tangent: liches, then, wrest the power of undeath from its chaotic roots and transform it into a kind of perverse order to defeat the natural entropy of mortal existence. The results of this, however, ripple outwards in a fractalized, chaotic pattern, once more serving the ends of destruction.
To keep the "D&D canon" a little distant, instead of referring to demon lords by their more familiar names I think cults devoted to them in Krevborna will call them by their more obscure epithets. Blood Lord for Orcus, the Sibilant Beast for Demogorgon, etc.
(3) - Asmodeus, chief of devils, is simply referred to as the Devil by the Church of Saintly Blood.
Also, it is interesting that the Monster Manual gives a genesis point for demons (they are spontaneously generated by the Abyss) but there isn't an origin attached to the devils. For now, I'm going with the fallen angels archetype.
(4) - Of course, this doesn't have to come into play just in the late game of high levels; it can also be something that texturizes the more obvious intrigues and power struggles in the setting as well. The vampires of the von Karlok family are essentially demonic; the Graymalk witches are essentially diabolic.