When the Elemental Four had stripped the surface from their world and started anew, they had thought that their discarded efforts were obliterated into nothingness. They were wrong.
Unbeknownst to them, even gods cannot entirely, utterly destroy matter. Rather, attempts to do so instead banished it into a demiplane - a sort of cosmological storage space - where it would wait for its time to be return to or be reformed into the world. This was the fate of the First World. The world of the Demons.
From the outset, the creatures that would come to be known to the mortal races of Teicna as Demons were flawed. It was intentional, really. What bored deity would want to watch perfect beings go about their lives? And yet, these early attempts were flawed in ways that offended the gods that created them. Beyond their free will and physical imperfections, the demons appeared to their creators as being almost literally driven by magnified aspects of the gods' own worst character traits. Greed, wrath, sloth, and many others were made manifest in these creatures, and the gods couldn't bare to look upon such caricatures of their own failings.
When their world was peeled from the surface of Teicna and discarded like the rind of a fruit, it was became warped and twisted, inverting into a hollow sphere, at its center appearing a sort of mirror of the core of the planet it had left behind. What water didn't leak out through the cracks in the inverse crust rapidly evaporated and suffused the atmosphere with a thick, persistent fog in the heat of this ever-present flaming orb
And so the demons were left without their gods, to survive as best they could in the barren hellscape that their once-beautiful world had become. For millennia they eked out a living and adapted to their new environment. For some species, this meant stooping to the level of mindless monsters, while others harnessed this hardship, steadily bettering themselves despite the predispositions that had made them so undesirable in the first place. By the time they made contact with the world again, the demons were all but unrecognizable as the beings the Four had discarded in ages past.
With the advent of magic amongst the sapient peoples of Teicna, more and more wild, chaotic energies were let loose into the atmosphere, and as these energies built up, they began to have strange effects on the world and its inhabitants. Most relevant of these effects to the demons was the creation of rifts. These extraplanar holes in the fabric of space connected Teicna with its lost predecessor in a way that would allow beings to cross from one side to the other. Undesirable as their current predicament was, it was only natural that the demons, when eventually made aware of what these rifts represented, would make every possible effort to be through to the other side before they closed again. 'Every possible effort' often including slaughtering any and all obstacles in the way.
For centuries, these escapees from the Inverse realm - frequently soaked in blood and half-mad from the efforts of reaching the rift alive - were the only contact the peoples of Teicna had with the Demons, and they could be forgiven for being incredibly fearful of them as a result. Between their incredible fortitude and their enraged status upon exiting a rift, Demons were known to rampage through countrysides and overpower entire militia forces before being taken down. This resulted in fairly standard orders to kill demons on sight in nearly every inhabited region, making it next to impossible for this impression of the creatures to change no matter how rational future entrants might have been.
When Malephesh tore the world in half and the waters of its oceans were banished to ensure the safety of the planet's core, they too found themselves in the world between worlds, pouring from the skies in two gargantuan sheets and wiping out a great many demons along the way. In time, they formed a great sea through the center of the demons' realm and brought liquid water back to them for the first time since their banishment. Now, the demons' bodies had not been nearly as dependent upon water as those of the current mortals of Teicna in the first place, and they had grown steadily less so over the years, but after being used to it only being present in the form of condensation in the air for so long, it felt to them like the nectar of the gods itself was raining from the heavens. With the sheer quantities pooling in the new ocean ring, petty squabbles for the resource rapidly became petty squabbles for the land beside it, which in turn led to the rapid formation of more organized tribes and governments centered around trading filtered water farther out into the wastes. Within only a few decades, all of the land bordering the sea for hundreds of miles out had become claimed territory of one clan or another. A few more after that and they had joined together to form Urba'at, the great megacity of the demon races.
In time, more reliable means of opening rifts between Teicna and the Inverse would be developed as the demons crossing over from the latter to the former became increasingly more likely to be civilized socialites rather than raving monstrosities, and soon enough entire contingents of demons were colonizing what Teicna's natives considered to be uninhabitable wastes, aided by their centuries of expertise with just such environments.
While they have been diligently working to clear away the prejudice that years upon years of violent demonic incursions laid upon their kind, the demons are still not entirely trusted by most Teicnan citizens. As such, they largely keep to themselves outside of the odd trading caravan or other professional travelers. With a few significant exceptions...
As demons are made, not born, they vary wildly in specific appearance based on the personal tastes and desires of their parents. Across long spans of time, some demons might become wholly unrecognizable as members of their species compared to more ‘contemporary’ styles. The only common threads between them as a collective species are their eyes, which are said to be dark voids with specks of light serving as their pupils, and various crystalline growths said to represent the strength of a demon’s spirit, colloquially called the demon’s soulstone. Both eyes and soulstones glow with an unearthly internal light, the color of which represents some aspect of the demon’s character, though records of what each color is supposed to signify are rarely consistent with one another.
Like their homeland, they don’t quite exist in the regular realm. Whether they’re in the World Within or the Brightlands, what is seen of them is only a small portion of what they are. It is their physical identity, shape, and an indication of what kind of demon they are, but part of them exists elsewhere, in a sort of miniature dimension that they carry with them.
For all demons, this hidden ‘Inner Self’ contains their mind and soul, allowing them to survive most injuries short of total obliteration of their physical forms, so long as the trauma doesn’t shatter their psyche. Slain demons will regenerate from most trauma, up to and including regrowing vast amounts of their bodies at once. Should their mortal-plane body be entirely destroyed, or should someone manage to attack and seriously wound a demon's Inner Self directly, destroying their soul's anchor to reality, they will finally be slain.
Nearly all demons store more than just their identities in their biological backups. Many have entire systems of muscles or nerves, excess stomach space, or even additional limbs within these spaces. What this means is that a specialized demon might be far stronger than its visual muscle mass would ever seem to indicate, or that their detached limbs might still be perfectly functional, or that something five feet tall might be able to swallow a cow whole without skipping a beat. Long story short, a demon’s appearance can be incredibly deceiving.
Due to how magic is accessed, drawing from either the physical or mental strength of the user, demons are able to channel their natural regeneration into being proficient, nearly limitless spellcasters. There are upper limits to the scale of the effect they are able to create, as pulling all of their strength at once can actually drain even their extraplanar body dry, but they can fire off smaller spells that they have mastered with incredible stamina.
As with mortal magi, demons must still study the magical arts, and many simply don’t have the aptitude or patience for the requisite training. Those who do, however, are often quite capable of picking up at least one school of magic and mastering it within a few decades. The especially courageous will even pick up two. There currently exist fifteen demons who have been both smart and strong enough to live for many centuries, becoming terribly powerful magi, with mastery of at least four fields of magic each.
Genetics and Reproduction
As mentioned above, demons are not born. Rather, two (or very rarely more) demons come together, craft a facsimile of the soon-to-be-child, and then imbue that facsimile with fragments of their souls. Through this process, the resultant child is formed already fully developed, with its appearance dependent both upon the substances it was crafted from, as well as the desires and souls of the parents involved. While many demons stick to having children of their own species - or else that of one of the parents involved if the group is mixed-race - there is nothing stopping the child from being formed as a member of any one of the established demonic subspecies, even if neither parent is a member of it.
Civilization and Culture
The demon language is the closest in the setting to something wholly unique, but what that currently means is that most individual demons’ names and things named by them are tentative and prone to change. The only significant exception at the moment is the suffix “-a’at”, which is used as a shorthand for ‘of the demons’. Seen in places like the names of their world and city (Doma’at and Urba’at), as well as their two major gods, it’s reserved for only the most significant and universal aspects of demon-kind.
Beyond that, with many demons dipping into and out of the mortal world, they’ve brought many things back with them as novelties, foreign languages among them. For demons born in the mortal world, or to parents who are fascinated by it, their names are likely to mimic the formatting and languages of the races that most caught the parents’ eyes.