Fiend

Fiends are denizens of The Abyss, their corporeal and spiritual essence permeated with its destructive and consuming nature. While they like to lay claim to primeval antiquity, in truth, they are no older than the Mortal Lineages. The first fiends were Abyssborn, spawned directly from the substance of the Abyss in response to the corruption of the Colossus War, and some are still born this way.
The first such beings were primordial and crude, but powerful, of the type known as daimons, or Loths. They were terrible and destructive, but lacking in strong opinions when it came to cosmic order. Soon, however, the corruption began to pool around the carcasses of slain Colossi, and under this influence, gave birth to the first Demons. These fiends were born craving the devastation of all things and the death of all law.
By this time, however, mortals had begun to multiply. Some of them had already found inventive ways to breach the harmony of the World and so damned themselves, and those who followed them, to a descent into the Abyss. Souls possess a certain gravity, which draws them up towards The Firmament when they are not grounded by a physical body. 'Evil' actions, those which go against the original harmony of the world provide a countering pull towards the Abyss. Conversely, devoted service to a God, especially a blue divinity, increases a soul's upwards gravity. The actions which create a downward pull include such obvious evils as violence, cruelty, destruction and oppression, anything which perpetuates an imposed hierarchy in place of the natural balance. The use of arcane Magic, and any action which transforms the world away from its original state can also have this effect, however. It is for this reason that artisans are often devout followers of craft gods, and wizards either hew to a cult of magic, or seek to reject the gods and mortality alike.
If the balance of a person's gravity is towards the Abyss, they are drawn down through the Grim Plain on death, to be washed into the River Styx. It is said that the first soul to follow this route, bypassing the promotion system that later Devils would undergo, immediately declared himself Ultima the Absolute, Sovereign of the Abyss, and set about defending the World - and most importantly their own chance of ever ruling it - from the depredations of the Pits. He began harvesting other souls from the dark waters to build an army. Most such souls were reborn as weak and ill-defined beings such as lemures, manes or larvae, but those of exceptionally long-lived or strong-willed individuals, or exceptional creatures such as dragons, might emerge as something greater or at least less common.

Fiendish Nature

Whether born directly from the Abyss or formed from a fallen soul, the corruption of the Abyss acretes around a fiend's fundamental essence to create its form. On the one hand, this gives the fiend a physical presence and agency, but on the other it subjects their essence to the corrosive nature of the Abyss. Will alone can sustain that essence for a time, but ultimately they require something else to protect them. 
There are broadly three types of fiends, differentiated by philosophical outlook, and the means by which they maintain their self, but they share a common anatomy.
The core, the self, of a fiend is their will and self-knowledge, a bundle of hard ambition, off-corrupted memory and determination that is known in abyssal lore as the 'curse.' In fiends dragged from the rivers, the curse is the relict of a corrupt mortal soul. The curse grows as the fiend becomes more powerful, but when they are stripped of rank or shown away from the Abyss, it shrinks, and the fiend is shorn of much of their past.
The curse is clothed in abyssal flesh, which clings to the curse in a mass known as the simulacrum. The greater the fiend, the more control they have over the simulacrum's form. Thus, lesser fiends have both cruder forms, and a more uniform appearance than their superiors.
The shape of the simulacrum is bound by the fiend's name. The name of a fiend is what defines them, as much as the curse within. Fiendish names are both a choice and a shackle, bending the nature of the fiend, but selected by them.

• Devils are creatures of law and order, bound about by pacts and contracts and obligations which protect their names and sustain their form and identity. They are consumed by a desire for control and power, over themselves and over others, which leads them to feel investment in the protection of the World which they hope one day to rule. The most powerful of their kind, the Archdevils, are even able to extend this protection to the Abyss around them, creating the pockets of stability known as the Nine Hells, each one anchored to one of the Pillars of Aiaos.
  Devils use their names to secure pacts, usually with mortals. The pacts give them power, and power allows them to forge more powerful names and thus secure greater pacts. Their strength, therefore, is tied to their ability to make and fulfil bargains, wearing webs of obligation which resemble a formalised version of a god's bond with their congregation.
• Demons care nothing for order. They are creatures of id and appetite, and as a result are always within spitting distance of dissolving into the Abyss. In order to prevent this, they gather names which they weave into ever-changing, constantly eroding, polysyllabic identifiers in which to wrap their essence. The Demon Princes who rule them cannot enforce order on the Abyss, but they do guide the chaos into something approaching patterns within the Pits which form around the carcasses of slain Colossi.
  Because they are not deal-makers, demons do not collect names as pact anchors; instead, they have a single, convolutedly polysyllabic name that serves as a shield against the corrosive effect of the very powers they serve. Their names are ever-changing, as syllables are consumed and replaced with ones taken from those they are able to overcome.
• Loths are mercenaries, craving above all that which is given willingly, but not happily. Like devils they are dealmakers, but unlike devils it is the payment received that sustains and invigorates them, not the existence of the contract itself. Sharp dealers and obsessively self-interested, they have no stable hierarchies, and because they do not commit to their obligations, even the most powerful among them cannot stabilise a domain on the scale of the Hells.
  Like devils, daimons make deals, but these are purely transactional. They gain no power from obligation, and so their names are instead their brand. Daimons lie as easily as they breathe, but they will never lie about their own name, nor will their name contain a direct lie. Consequently, daimons choose names which advertise their most decisive and saleable traits. These names are typically deed or virtue names.
  Should a daimon give a false name, answer to a false name, or betray their name by acting against its terms, their name is broken and the daimon reduced. For this reason, daimon names are also often both vague and affirmalive. Arragh Foesmiter only needs to have smite foes, not to smite every foe, whereas Luggub Never-Bested would lose their power after one unlucky game of dice.