Demon

In order to become a demon, a creature must be infused with fel energy. As the existence of non-sentient demons suggests, a creature's wishes are irrelevant to the process. Willing or not, a creature can be changed into a demon. Such a transformation alters the affected creature on a fundamental level and typically leaves little or nothing of that creature's original personality.

Background
Before life even began, before even the cosmos took shape, there was Light ... and Void. In the form of a boundless prismatic sea, the Light swelled across all existence. The ocean of Light was dynamic and ever shifting. Yet as it expanded, some of its energies faded and dimmed, leaving behind pockets of cold nothingness. From the absence of Light in these spaces, a new power coalesced and came to be. This power was the Void, a dark and vampiric force driven to devour all energy, to twist creation inward to feed upon itself. The Void quickly grew and spread its influence, moving against the waves of Light. The tension between these two opposing energies eventually ignited a series of explosions, rupturing the fabric of creation and birthing a new realm into existence. In that moment, the physical universe was born. The energies released by the clash of Light and Void raged across the cosmos, matter merging and spinning into worlds without number.

The Rise of Twisted Souls
The most unstable energies coalesced into an astral dimension known as the Twisting Nether. The creatures that had emerged from this realm were known as demons. They had been formed as a result of the Light and Void energies that had bled together at the borders of the Twisting Nether. The demons embraced their furious passions and reveled in pushing the boundaries of their power, heedless of the consequences. Many of these abberations indulged in the energy that pervaded the Nether. Some learned to wield the all-consuming powers of fel magic and, before long, the bloodthisty demons clawed their way into the physical universe, terrorizing mortal civilizations and bringing ruin to world after world.

Subraces
Abyssal

Demonic hound

Ered'ruin Floating eye (Beholders)
 * Doomguard
 * Doomlord
 * Fiend Eater

Imp

Infernal

Pit Lords

Mo'arg Nathrezim Sayaad Voidwalker
 * Felguard
 * Gan'arg
 * Tothrezim
 * Succubus
 * Shivarra

Void terror

The change to a demon being includes a powerful command of arcane energy. Similar to sorcerers and wizards, demons control their power with the force of their will, but their magic is innate. The demonic magic that infuses them also confers certain immunities and resistances (acid, cold, electricity, and fire among others) and they radiate an aura that induces fear into the weak willed.