Firmament - General Overview

Reality exists in layers. At the 'top' - the zeroth layer - is the infinite primordial chaos. The first layer is the land of the living. The second layer is the land of the dead - or, at least, those who have died only once. There are more layers beyond the second, each increasingly mysterious.
There's 'chambers' between each set of layers. The chamber between the zeroth layer and the first is the River of Stars, where the gods that simultaneously threaten reality and preserve it dwell. The chamber between the first layer and the second is the River of the Dead, which the deceased must travel through and along to reach the door to the second world.
The dead whose bodies are destroyed before they reach the second world become unmoored. Young children, not settled into their personalities yet, will gain amnesia and easily reincarnate. Adults usually become stuck as ghosts. Being a ghost is dangerous - some things can destroy or alter your memories, your personality, or your very existence.
The journey to the second life is also dangerous. The River of the Dead is full of hazards, many of them undetectable to the untrained eye (or undetectable at all, meaning you must memorize where they are, or bring a map with you). Most of the dead therefore make the journey in groups, lingering at the beginning of the river until their appointed guide dies (most people decide who they will follow before they die). Appointed guides can be wise women, story keepers, priests, nobles, or royalty. They are always well educated about the afterlife.
At the end of the River of the Dead sit the Judges. There's debate about who they are, how many they are, and what their personalities are. The general agreement, though, is that they judge souls harshly for dishonesty and disrespect (of nature/ gods/ the dead), penalizing them in the second life - or destroying them outright. The Judges can be avoided, tricked, or persuaded, though - one of the tasks some cultures assign to the guides.
Magic comes from two places - the gods and the dead ("low" magic) or the stars ("high" magic). Star worship and magic is frequently illegal (the Judges of the Dead severely disapprove of it, and of rulers who allow or encourage it). High magic is tied up in the primordial chaos, in chance, and in the breaking of patterns, and is mostly based in the user's own power. Low magic mostly consists of calling on another to intervene on your behalf (praying to a rain god for rain, asking a ghost to cause misfortune).
Sometimes the worlds partially intrude on each other. The dead may bubble up from the underworld, or items may fall from the star-chambers. It's immensely rare - almost mythical - for two distinct layers to touch, though; usually it's the spaces between them that get a bit blurry.