Demoniad (Part 1)

Demons are perhaps the foremost antagonist in my D&D setting. I conceptualize them as a blend of traditional chaotic-evil D&D demons, Warhammer's chaos daemons and Lovecraft's malevolent Outer Gods.

Things fall apart; the centre cannot hold;

Mere anarchy is loosed upon the world,

The blood-dimmed tide is loosed, and everywhere   

The ceremony of innocence is drowned;

- The Second Coming by W.B. Yeats

The expanse between worlds is not empty. Compressed into the spaces around coherent realities, demons fill the non-places in infinite numbers. They thrash and batter at the boundaries of existence, pressing in greedily, eager to corporealize and burn the world.

Demons are the incarnate manifestations of chaos. They hate structure and constraint above all else. The only thing saving the multiverse from overwhelming annihilation is their inability to organize. 

The Blind Idiot

At the center of everything, at the bottom of the universe, there is Azathoth, the all-consuming, blind idiot god of Chaos, incapable of perception, awareness or purpose. He is the ultimate well of entropy, a mad hole of utter un-forming. Reality unwinds as it approaches him. Multiverses slide into his maw and disintegrate.

Azathoth's frothing surface ceaselessly sloughs off emanations. They race in all directions fleeing their progenitor. These are the demons that infest all realities. This infinite horde is possessed of all his madness and hunger. They gnaw at the foundations of reality and hasten its collapse.

Manifestation

Demon souls are willful but their disorderliness makes it difficult for them to manifest and navigate in the physical world.  Their consciousnesses, such as they are, are often too focused on disintegration to build a body capable of moving in reality.  

But demons are nothing if not malleable. Without a fixed form of their own, when they manifest into a physical world they change. The form of their consciousness flows and reforms to fit the shape of their new body and they become a little less demonic.  They absorb foreign concepts like object-permanence and cause-and-effect. They develop memories and goals. They learn to wield power and subvert structure. 

Possession

Rather than gathering matter and congealing bodies of their own, demons often find it easier to take on existing forms. 

• They might whisper into the minds of men and find hosts willing to share a body in exchange for power.  Usually this only results in madness and inefficacy for both, but sometimes the human and the demon find common cause and a chaos sorcerer is born. Other times the human consciousness is fully devoured and the body warps into an unrecognizable parody.

Pink horror

• The recently vacated husks of the deceased are comfortable vehicles for those not constrained by common biology. This is the source of most of the "mindless" undead that plague caverns under the earth. 
• Stronger demons can find purchase in inanimate objects. 

Summoning & Binding

In their hubris, conjurers invite demons into reality, hoping to use their boundless energy to accomplish great works. With cunning, this can succeed. 

When a conjurer drags a random demon forth the from the immaterial Outside, they are a mindless monstrosity, shifting in shape and reflexively hostile. In this form they are outrageously dangerous and nearly impossible to use. With care though a demon can be enslaved.

Masked Demons

The most common way to enslave a demon is to bind them with a mask and a name. This grants them sentience, memories, and comparative physical stability. Clever design of the mask can determine the form the bound demon will take. If the binding is properly applied, knowledge and use of a demon's name grants tremendous power over them. 

Kill Six Billion Demons

Slavery is the ultimate anathema for demons. To be bound is absolutely contrary to their nature and they buck against their fetters with every spare ounce of energy. Thus it is essential to keep a bound demon constantly exhausted with unending toil. Otherwise, it might find a chink in it's chains and break loose.

• If its mask fits sufficiently tightly, a demon can be forced to toil so forcefully, that it consumes itself. This is the safest way to dispose of an unwanted bound demon. 
• Rarely, a mask will re-mold the demon it constrains. The creature will learn to value their personality and dread the dissolution of personhood that would result from an unmasking. 
• Over millennia of practice, demonologists have devised a number of standardized mask types.

K6BD again.