I think I'll do freeform with some background statistics to determine certain outcomes with more standardization.
As far as races go, you have one option: Human. What you do with demons/undead/monstrous humanoids is certainly your business and may affect your character physically, but you'll start out (if not end at) Human. If for some reason the opportunity for a demon/whatever PC comes into play later, I'll discuss it with them, but that's for later only.
On the planet Skorn, this game will be taking place in Nadir. The greater continent where Humans reside is split down the middle by a giant mountain range named the Behemoths; this mountain range is rumored/known to contain monsters and dangerous supernatural even before what went down, so no one in their 'sane' mind decides to head toward them. Nadir is the land East of the Behemoths, where the Humans first arrived on their world. The clime you are in is a mix of temperate grassland (Southern end) to temperate forest, with pockets of hilly land, swamps, highland, and turns into more arctic environment as one goes North.
Origin of Humans: It is a widely known legend of fact that the Humans here did not begin here. Once, over two centuries ago, your ancestors dwelled on another land, another world entirely; it is by the machinations of denizens on this world, known simply as the Ancients to your people, that Humans were abducted into Skorn through a portal of some sorts. On Skorn, Humans were the slaves of these Ancients, who were wiser, taller, stronger, more elegant beings whose life spans were measured by generations of Human ones. These Ancients regarded Humans at best as amusing toys and useful labor, while at worst pathetic creatures and vile scum. The course of time ground on, and the Ancients acquired more and more Human slaves from their world until the activity of this portal drew attention: one day, something else came through the portal that had never been on Skorn before.
A Demon. Demons came to Skorn from this other world, and the Ancients had little idea how to stop them. After the first, a few more came, and then hordes of them burst forth into Skorn, sweeping its lands under a tide of chaos and blood. Humans divided into groups, some running off into the wilds to hide, some siding with their Ancient masters, and others joining with the Demons. As the Demons marched even, other terrifying creatures came from the portal as well, most notably the Undead, which then sought to war with the Demons as well.
In the end, it was the Ancients that one, but at the cost of their race. Their kingdoms broken, their people slaughtered, enslaved, fleeing, or wounded, they made a pact with some power and banished the Demons from this world. Or so it was thought.
The Humans of the land wise enough to hide throughout the conflict were the only ones left in Nadir, and came out to dazedly claim the land and build their own civilizations, making use of what little of the Ancients wasn't pillaged or looted. And thus human civilization grew, their acquisition and industriousness leading to them spreading through the land in the course of a few years.
Despite several civil wars in time, the Humans largely prospered. This prosperity became threatened a year ago.
No one understood how or why it happened, but not only a Demon, but a dozen-membered Demon Cult made its nature known to Nadir after getting caught sacrificing citizens of a far Eastern town. A rash of demon worship spread from this incident, as it was made known to Humans that power could be obtained through an oath and the bidding of something greater...
The hearts of those tempted by darkness have twisted society's prosperity, and the wilds began to be taken by evil.
This is the world you are making your character into.