The Afterworld

This is a world that all things on Earth appear in when they die. This isn't an Afterlife in the typical sense, since you can die again from all the same things... in addition to ways that were impossible on Earth.

Aging, reproduction and evolution are also the same as on Earth, although evolution here can create much stranger life due to the Afterworld being home to a special kind of energy. Most call this energy magic, despite how much that would make the average scientist stomp their feet and cry and give flamethrowers to children while swallowing pieces of uranium as comfort food.

Magic can merge with substances to alter their properties like hardness or magnetism, functionally making whole new substances, and be turned into energy. It can be obtained by pulling it from substances it's been merged with, reverting those substances back to their original states, or made from energy. It can only be affected by and affect things that are in contact with it.

Most of the Known World is uninhabitable or nearly uninhabitable for normal humans, instead having people who've evolved special traits or no people at all. There are some nice places, though, the largest being Lamara. Lamara gets the most focus because it has the most internal variety and people, plus a lot of the story set in this world takes place in Lamara.

Also, Understomachs. Most Otherworlders appear already inside of Understomachs. Right above oceans of acid that they almost immediately fall in and melt inside of.