The world of Nibirion looks familiar enough for a fantasy setting in most respects. Below are short worldbuilding questions and answers that mark out its key features quickly, without going into detail or secondary nuance.
What happens to mortals when they die?
Creatures without a soul simply die.
Creatures with a soul release it when they die in their own plane – the soul leaves the plane and enters the Void, where it wanders until it meets the Roots. Along the Roots it can reach another world and be reborn there in a new creature (possibly even return to the world it came from).
The Abyss of Spirits, passing through the Void, may meet a soul and seize it. (Most likely they will simply devour it.)
A soul can also end up in the Abyss (of Fire or of Ice) or in the world of the gods. That depends on whether its owner sold it to demons or to gods (both sides are after as many souls as they can get).
When a demon, an angel, a Varkh or even a god perishes, its soul is likewise released and travels the same road as a mortal soul from the material plane. (The only difference is that nobody in the Void cares about a mortal’s soul. Over the soul of a mighty being a whole battle may break out.)
A note — on souls, and on the souls of great beings.
A few interesting conclusions and examples follow from this.
For instance, the Void holds a countless multitude of souls of great beings, no weaker than gods or demons, who simply have not yet taken flesh again after dying in their own plane.
Or another example: in some backwater goblin khatso, an infant may be born… with the soul of a mighty demon.
What is unusual about the shape of the world?
The world is a planet in orbit around a star. What sets this world apart is that its star systems are joined by the Roots, which allow travel between them at incredible speed.
What fear do mortals share the world over? What do they do about it?
There is no single fear that unites all mortals. Most people are busy with everyday concerns, and their worries are earthly enough: hunger, sickness, the death of those close to them, the loss of the soul. Some seek protection from the gods, others rely on magic, and others simply hope for tomorrow.
How do people travel long distances?
The most common way to get about is riding animals and carts. Wizards use teleportation portals, which are open to very few. Large cities run public portals linked into a wide network. The system resembles air travel in our world. Regular routes, transfer points, restrictions on who may use them.
Which landmarks stand as reminders of an important event?
The Castle of the Rune King is one of the oldest structures in the world. Scorched land all around it, towers climbing to the sky itself, crimson sunsets reflected in obsidian walls. It is the work of the high Eternal god Iniax, raised to hold back a demonic invasion. To mortals, though, the god’s avatars and servants look more like tyrants, and so from time to time armies march on the citadel.
If you are interested in finer nuance and detail, there are deeper sections to explore:
The Currency System — a short overview of the coins that exist in the world and how they are used across the different kingdoms.