In the Ancient Lands are several other world than the one which is home to the humanoid races, but compared to many other setting, the entirety of all these realms is relatively simple. There are three main realms: The Material World and the Spiritworld, which form a pair of two mirroring, but not quite identical realms, and the Void, the endless and eternal space that lies beyond them. The Material World and the Spiritworld are not eternal or unchanging, and there might be countless others like them far out in the Void. This even seems very likely, though nobody has ever been able to find anything that would prove it.
The Material World: It’s the world of physical matter and mortal creatures. By itself this matter is lifeless and inanimate, but both the forces of nature and the spirits if all living things are maintained by life energy that comes from the Spiritworld. Most of the Material World is an almost empty space in which countless numbers of stars and planets exist. Many of which are lifeless rocks, but there could still be many thousands covered in plants and wildlife. However, even the most powerful magic rituals can not allow a person to travel between them.
Spiritworld: On a first glance, the Spiritworld seems almost identical to the Material World and might even appear indistinguishable from it until obvious signs of its magical nature are encountered. However, in reality the Spiritworld consist entirely of magical life energy that takes solid forms almost, but not completely mirroring the environments of the Material World. Every tree, mountain, and lake has a counterpart in the Spiritworld, even though they might not look the same or be in exactly the same places. Events that affect the spirits of a place might be invisible in the Material World, but can lead to severe changes in the Spiritworld. Animals and humanoids are the exception, as their spirits exist entirely in their material bodies and is separate from the Spiritworld. Furthermore, all natural forces are much more powerful, including the strength of wind, the heat of the sun, and so on. This makes travels to the Spiritworld highly dangerous to mortal beings, as their bodies are not made to deal with such forces.
Underworld: The Underworld is not actually a separate realm of existence, but rather a different region of the Spiritworld. While what most people are calling the Spiritworld is the mirror image of the surface world, the Underworld are those regions that are mirroring the inside of the planet. Since the Spiritworld is not an exact match of the Material World, there are vast systems of interconnected tunnels and caverns that have conditions that could be survived for extensive amounts of time by mortal creatures with magical protections.
The Void: The Void is quite unlike any of the other realms. It’s the space that exist outside of the universes, and concepts of time and distance have no meaning there. While physical matter can be brought to the Spiritworld and Spirits manifest in the Material World, matter can not exist in the Void and is confined to its own universe. However, the Void does have energy and some sorcerers have managed to separate their souls from their bodies and take short peeks into the Void. At the beginning of the universe, an infinitly small fraction of the Voids energy started to form into matter, creating with it space and time, and eventually all life as it exists now in the Ancient Lands. Eventually, after billions of years, space, time, and all matter will again disintegrate into the primordial energies it was made from and return to the Void. This has happened countless times before, and will happen countless times again. Not just one universe at time, but a potentially infinite number, as the Void has no beginning and no end.