On Chaos

This post will have very little in terms of gameable mechanics. I'm wary of posting too much that borders on "game themed prose fiction" but I intend to refer back to this stuff in later posts as I share my campaign cosmology.

From hereon out, I'll tag this kind of thing 'fluff'. This sort of thing never comes up directly in a game, but its a "headcanon" that informs the game. 

Chaos isn’t a party. It isn’t unbounded freedom (at least not for long).

It’s the ground state of creation. It’s a bonfire followed by a heap of ash. It’s the last photon in the universe decaying, then an infinity of darkness.

A forest seething with life and filled with predators and prey is order. An empty, crumbling parking lot is chaos.

(Zdzislaw Beksinski)

A town filled with looping side streets and poorly marked cul de sacs might feel "chaotic" in the colloquial sense (at least if you’re a stranger to the place). But a district of tightly gridded, identical apartment high rises with identical units filled with identical furniture is an environment where chaos reigns in the cosmic sense.

On the right: Chaos (Photo by Ursus Wherli)

Anyways, you see what I’m getting at.

Demons are chaos, forced by foolish mortals into a fleeting prison of flesh, because pure Chaos simply can't exist in our universe of ordered matter and energy.

Demons don’t hate life and order; that would require something akin to passion. It’s just that everything that exists (including themselves) causes them pain and misery—the kind that human nervous systems don’t have the hardware to comprehend.

They endure their own miserable existence only because they have no means to end it, and once they’ve been summoned, they seek any and all means to hasten the return of the universe to its normal state. During their tortured birth into the material realm, a horrific arrangement of organic forms, concepts, and nightmares-made-real cling to them like dust clinging to a bead of water.

To decide what that looks like and what it does, you could use Appendix D at the back of the 1e DMG. Or make it a Paradox Beast. I tend to just stick whatever traits I like on board. They haven't come up often for me to come up with a unique thing for them mechanically.

With regards to PC alignments...

Neutrality is just near term pragmatism.

Lawfulness describes, simply, a spectrum of preferences for order—even a tyrannical or capricious order—over chaos.

If you want some ideas on how a truly Chaotic NPC looks at the world, look no further than Thomas Ligotti.

When asked by an interviewer what would make him smile, Ligotti replies:

“I think that if I could walk from one end of the world to the other and see nothing but annihilated landscapes, that would make me smile.” (source)

Warhammer Chaos Warrior (it's probably sacrilege but I don't know the illustrator! Let me know in the comments and I'll add a proper credit)

 These guys aren't Chaos. They're just gasoline and a match.

Demons of the Outer Dark

Outside the Aether is just black, empty void. Not even air to breathe.

By Agostino Arrivabene

No. That’s wrong. It only seems that way because of your primitive instruments. You know, the ones rolling around inside your skull.

Out there is primordial chaos. The monsters that the gods cast out before time began. Call the tiniest whisp of that stuff into the material plane, and it congeals into a corporeal form known as a demon. Being forced to take on a material body appeals to it about as much as being pressed through a red hot sieve appeals to you. It reacts to your summons accordingly. That’s just one reason why you’d be wise to keep the thing contained with strong magic.

Ades Pantocrater - Agostino Arrivabene

Magic circles and the like are often effective just because they’re a complex, orderly geometric pattern. Any kind of organized structure might be sufficient to send the demon’s physical form into a crash state. Or maybe not.

Killing the physical vessel doesn’t free them, though it might disperse weak entities enough that they can’t reassemble their physical form.

They must be banished with tools mortals cannot create themselves (certain rituals and items handed down by transcendent entities, cleric spells). The disintegrate spell also annihilates demons permanently, creating a small amount of Black Aether*.

It's hard to say whether demons exist as discrete entities outside the material universe, or if their individual nature manifests only when drawn into our world. It's possible that the ritual and the sacrifice itself is what shapes raw chaos-stuff into a physical form.

While their powers can be terrifying to behold, they are summoned and tormented most often for knowledge. Their realm exists outside and throughout the material universe. It's likely they know more than the gods themselves.

http://www.agostinoarrivabene.it/paintings/

While a demon may take on numerous forms, and manifest subtly different powers with each summoning, it's one pleasure on the material plane is to degrade order into disorder. Set loose from its bonds, it endeavors to turn families into broken individuals, orchards into ashen waste, and kingdoms into chaotic orgies of death and destruction followed by silence.

It has no name, but you can be certain it will always remember you. It's fondest wish is to return the favor of its summoning, and bring you home as a souvenir.

*(I don’t know what that does yet, but it will probably be something like a metaphysical version of this. Highly reactive in unpredictable ways, with precipitates that might have a variety of effects on the material world).