Demons

"They flay men, draw out the entrails from women, and drive spikes into the joints of children. Show them no mercy, for they have none." - Captain Fyodor Petrovich

Some say that power always corrupts.

A being is composed of different components, some physical and some immaterial. Mortals balance the material and the immaterial in a way that allows them to choose their own destiny. Supernatural beings have an immaterial aspect that guides their behavior and grants them their power. Though they can often overpower mortals without great effort, they must follow the narrative role given to them by their aspect. Demons are dominated by a concept that has been twisted to its destructive extreme and intensified into a painful compulsion. Typically, this is called a vice. Though it resembles an incarnation's virtue or a celestial's ideal, it forces itself, crushing the demon's soul into a vestige, forcing itself into every thought and event around the demon, and disrupting the balance of everything it can. Aspects typically act as a specific theme in a story, but a vice attempts to control the story and push out free will. When an undead's hollow acts as an addictive compulsion, they can sate their cravings for a time. But a demon is both compelled to obey their vice and to impose their depravity on everything they encounter. For example, a celestial who holds the domain of war must champion causes, associate with warriors, or encourage combat, but a demon enslaved to conquest must make war for war's sake, unable to tolerate a time and place not embroiled in destruction or a person who is not a soldier or casualty. A demon's vestige, if it is particularly strong, can pick the more "fun" or safe ways of forcing their roles on others, but many lack even the personal choice to decide how they pursue their vice's goals. This need to make other beings follow their aspect is what gives fiends their reputation for corruption.
Because a vice is a twisted form of a virtue, ideal or hollow, the possibility of a fall to demonic corruption exists in all beings that dedicate themselves to an aspect. "Evil" aspects are much more likely to corrupt, like greed or hatred, but neutral motifs like fire or lust also have the potential to turn depraved. A "good" aspect like justice or love tends to pervert; a demon of romantic mania is a twisted form of a spirit of affection. An incarnation can slip over the line from virtue to vice, little by little until they reach a tipping point. Corruption in the early stages is invisible and reversible, no more than a passing mood to an outside observer. But after a certain point, it begins to affect the incarnations' powers and even their physicality. At this point, an incarnation might develop mutations, deformities, or a fearsome aura. Celestials whose aspects naturally rule a greater part of them and whose ideals are more powerful and less flexible are especially fearsome when an ideal corrupts. Fallen celestials sometimes carve a destructive path before they even realize they have become demons due to their powerful devotion.
This process can also affect mortal spellcasters. Sorcerers who perform their modifications on the body must take care not to introduce corruptive elements which can affect the user's judgement. Even worse, witches and cultivators who impose aspects on their mind and spirit have the same diabolic risks as incarnations should they allow their aspects to control them. "Dark" magic is the colloquial term for the use of domains and motifs that easily twist into vices and for the actions of mages who have allowed corruption to take hold of them. Spells that flirt with corruption can be addictive and detrimental to a spellcaster's mind. Plain, mundane wickedness is more common for mages than demonic cultivation, but the former often leads to the latter, even if it is a process of decades.
Demons are immortal, but in a different way than celestials. They require an anathema, an aspect to counter their vice, in order to destroy them permanently. Exorcists unable to provide the correct anathema will resort to destroying the demon's body. A demon, when unbodied, does not naturally reform overtime and their vestige can fade if unbodied for too long. To return, the demon's vestige must draw aether tainted by their vice in order to recreate its physical form. To that end, they are drawn to leylines, nexuses, and spells that resonate with their type of energy. This gives the illusion that certain vile acts cause demons to spontaniously appear - demons are born from souls as are all beings, but places of sin or obsession attract unbodied demons which regain physicality as they bathe in the power. Unbodied demons often attempt to hasten this process by speaking to and influencing mortals who have the power to embody them, seeking the wicked and vulnerable and disguising themselves as intrusive thoughts, friendly spirits, or divine revelations. Most mortals are not open to this type of influence, but those who are can be decieved into committing vile acts or worse, opening their own bodies for demonic possession. Those with an affinity for a specific vice are potential targets for demonic influence; the more they indulge and obsess, the greater act of will required to fend them off.
Naturally, demons are hated and feared by most cultures. Warriors gain renoun by slaying them and exorcists work to hunt out their more subtle influences and drive them out where they lurk. Less scrupulous, power hungry folk may work in cohoots with demons. Many of the more personable ones with less violent vices can be negotiated with, and the less powerful ones bound to order a master's command. Some mortals who give themselves over to vice can let unbodied demons share their flesh or invest them with vice, gaining the physical changes of corruption. Be warned: even a demon that appears friendly or servile cannot control its vice, and will always be planning out how to express that aspect in the most harmful way possible. Some say that a demon can be redeemed or purified. There are few attested examples of this: simply destroying the demon and allowing the soul to be reborn as something else is the safest way to help them. It may be possible to return a fully corrupted vice into a controllable benign aspect and free the demon from the torment of its own depravity. Corrupted incarnations, diabolic cultivators, and tainted witches are much more easily saved, and some churches practice types of purifying cultivation techniques for preventing them from falling beyond redemption. Demons are not responsible for all or even most of the malice and sin in the world, however, as long as those aspects exist, demons will form and trouble mortals.