"Who Wants To Be The Demon King"

A simple fact is known to all of Demonkind; Demons are creatures of Myth, Magic, and Curse. Demons of all Clans are a culmination of magical energy formed from the defeat and murder of the original Gods of the world, now known as the Fallen Five, the Dark Gods, or just the Dead Gods, by the current Gods of the world, also known as the Goodly Gods, the Twelve Gods of Light, or just the Gods of Man. Their dying curses rang out through the world as that which could not be killed was felled by the next Gods of the world, and cursed the lands with strife, and Evil, for the sin of betrayal. Wherever their blood fell, wherever their viscera painted the landscape, Corruption - Demonic Power - took root. And beings that dwelled within those lands became warped, twisted by the powers of the Gods, leading to the creation of the first Monsters, and in time, the first Demonkin.

Demonic Energy is the basis of every monster and demonic creature's existence, whether they are mutants of natural creatures created from long term exposure, be they created with magic, spawned into the world by miraculous chance, or born naturally. It flows through a Demon or Monster body in a similar way to how Mana flows through all living beings, though the energies do not conflict. Most areas rife with Demonic Energy are unsettled by the Species of Man, even during the Golden Age of Man, when all the kingdoms were still United, for the corruptive influence can only be stopped temporarily even by those favored by the Goodly Gods. And it is these places that the Demonkin predominantly live, pushed into the far corners of the world, into the dark places no one else is willing to reside, in order to survive.

But Demonic Power doesn't only stem from ancient battle fields or places of godly violence. Every Evil act in the world is an act of the Dead Gods' curse. This energy is useless to most, though wizards on Dark paths might trap the energy for use in artifice or spellcraft, and those who follow Demons may offer it up to them in the moment of act. For Demons, this energy is a reinforcement of their existence. In each evil act they commit, they enforce upon the world their existence, and through the expenditure of its raw form they can reinforce Who they are, gaining enlightenment from the dark curses of their progenitor Gods. However, the use of Demonic Energy only allows a Demon to reinforce a trait they're already in possession of; they cannot gain new knowledge from reinforcement of their Demonic Existence.

However, Demonic Power is far from the only way Demonkin grow in strength. As being of Myths, the act of being known, of being spoken of, of being feared, and of their deeds being retold to others strengthens them as well. Infamous Deeds are a track of myths associated to their existence, tales told to the world about a Demon, and as these tales spread, they also become a part of the Demon's power. A demon renowned for besting a great warrior with their barehands will find their hands hardening into deadly weapons over the days to follow, as the deed spreads. This can lead to explosive growth in any Demonkin who experiences a great, public success. And those skilled with the manipulation of words might manage to fine tune the details of what is spread - though only a few Demon Clans do so regularly, most notably Succubi and Vampires.

However, Infamy is a double edged sword - for the more news that spreads of a Demon, the more eyes on them. A demon must weigh the risk of being known against the temptation of growing stronger - for a Demon with many an infamous deed of great magnitude will attract a far greater - and more tailored - response than an unknown Demon without a single Deed. Some would-be clever Demons make use of this negative aspect, however, utilizing their slowly growing Infamy to claim ever more Infamous acts, as their opposition strengthens. Few survive this.

And, of course, Demons are capable of learning through the simple acts of spending time; rote memorization, guided practice, and mystical sources of knowledge are still something a Demonkin can benefit from. Though some clans, most notably the Gnolls and Goblins, do struggle more with learning through practice than others. Not all Demonkin are created equal, a fact that the Demonkin themselves know quite well.

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TL;DR:

Three advancement resources.

Demonic Power: Gained from doing acts of Evil or having Man followers do evil acts in your name. Can only raise your current traits, cannot buy new ones. Also can be absorbed from evil artifacts.

Infamous Deeds/Infamy: Gained from doing big, notorious acts; as stories spread, you gain or improve traits related to the Deed. Also causes more people to know of you, for good and ill.

Time: Spending time practicing a skill over and over again, getting lessons from a skilled teacher, or reading mystical ancient books can gain or improve traits. However, Time is a valuable resource.