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.
Worlds
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.
Spirits
Nature Spirits: Most spirits people are dealing with throughout their whole lives are native to the Spiritworld and correspond to the landscape that surrounds their homes. When the people of the Ancient Lands talk of spirits, it’s usually these spirits of nature they are meaning. These spirits fall into two categories, place spirits and free roaming spirits. Place spirits all correspond to a specific feature of the landscape, from mountains and forests to lakes and rivers and down to individual plants. The greater a spirit, the more aware it is about its surroundings and the more it is able to communicate with mortal creatures. The spirits of small stones or flowers usually lack anything that mortals would recognize as consciousness, while the spirits of mountains and oceans usually don’t even take notice of such insignificant things like villages and ships, let alone individual people. Free roaming spirits have a much closer resemblance to people and animals, possessing a clear form and specific body that can be both captured and even killed. However, they are not made from physical matter, but from life energy that has taken a solid form, and therefore are not affected the same way by either weapons or the elements. Only silver and magic can harm a spirit for sure. Free roaming spirits include naga, shie, oni, and mermaids. However some spirits, like nymphs and spriggans, often appear in forms that are very similar, but are in fact place spirits that can create and dismiss such bodies at will.
Old Ones: Like nature spirits, the Old Ones are native to the Spiritworld and are almost identical to them in essence. However, while the nature spirits belong to the surface world of the Material World, the Old Ones are the spirirs of the Underworld. Life on the surface is always changing, just like the landscape, and so its spirits are also changing or disappearing and replaced by new spirits. But deep below the ground of the Earth and the surface of the oceans, the world is changing very little and only very slowly. Matching that, the spirits of the Underworld are still very similar to how they have been at the very beginning of time. While the spirits of the surface are unknowing in their thoughts and motives, people are at least familiar with them and understand how to please them and avoid their anger. The Old Ones, however, are completely alien to the minds of mortals.
Demons: Demons are in many ways very similar to spirits, but they are beings born from the raw primordial energies of the Void. They are truly eternal, though as beings of a realm without either time and space, it is difficult to imagine what that would even mean. While their numbers are probably infinite, only a small number of them seems to be interested or even aware of the material world at any given time. As time and physical matter are completely foreign to them, they sometimes desire to experience them firsthand. However, since they lack any physical form, they can not usually do so, unless the magic of mortal sorcerers prepares them a temporary shell like those of spirits, or they use the magical energies of the void to alter a sufficiently sized heap of ordinary matter to serve as a vessel. In theory, that vessel could also be a living creature, though the changes inflicted on it are irreversible and its original soul is entirely absorbed into the demon forever. In either case, the demon requires extensive assistance from a sorcerer in the material world and thankfully such cases have been very rare. As the energies of the Void are completely foreign to the Material World and the Spiritworld, the mere presence of a demon has a highly disruptive effect on both worlds, similar to the use of extensive sorcery.