All beings exist in the multiverse, vast and infinite amounts of worlds. They are widely different and varied. However to those of the Overworld, the Everglow is the only known other realm.
The Everglow is a realm where time and space are unconstructed, and home to demons.
It is possible for demons and objects of the Everglow to pass through the midworlds into the Overworld, however doing so requires a lot of magic and energy. Beings and objects that leave the Everglow can not return. Because the Everglow innately has more magic than the Overworld, it makes it much easier for beings to pass through to the Overworld. They lack the power to make a return trip.
Demons are the native inhabitants of the Everglow. They have no innate physical form or traits that relates them to other demons. They can come in all shapes and sizes and are inherently magical and immortal. They are not born in the traditional sense, but rather are created in nebula-like structures that litter the Everglow. They come into being as if waking from a dream, both anew and everlasting.
Despite the name, they are not necessarily malevolent, instead demons are known for looking out for their own interests. They operate on levels and morals that mortals don't comprehend.
Mortals cannot physically enter the Everglow as they lack the power to make the trip and survive the stay.
Some gifted individuals can use magic to tap into the midworld between the realms. They can meditate and have their mind enter the Everglow, this is known as transcendence. This is extremely powerful, but also leaves them extremely vulnerable both physically in the Overworld, and to their spirit in the Everglow.
With their mind and spirit in the Everglow they are granted as much power as their mind will allow, however they have a limited effect on the world and its physical properties. Individuals can communicate between each other in the Everglow, and use it to reach higher understandings of the sister world and magic in general, however the high presence of magic itself and the completely new world can be disorienting and hazardous.
Undergoing transcendence leaves your body vulnerable, and so it is rarely done alone or in dangerous environments. Death in the Overworld during transcendence leads to a complicated afterlife. On most occasions death of the physical body will lead to a death of the separated spirit, however it may shatter the spirit into fragments which will then burn out their energy over a period of time stranded in the Everglow. "Death" of the spirit in the Everglow however is a very different process as the death of a body and soul does not happen in the same way, allowing the spirit to take extraordinary amount of damage with little lasting effects on the individual. Depending on the mental and spiritual capabilities, one may undergo a lot of trauma while remaining in the Everglow, otherwise most are just shocked back into their bodies back in the Overworld.
Highly advanced magic users can absorb magic with their spirits in the Everglow, and use it once conscious in the Overworld, as a supernatural magical reserve. Even more advanced users can split their mind and be present in both the Everglow and Overworld simultaneously and be free from the normal magic restrictions, making them exceedingly powerful but also mentally distracted, and presenting the possibility that the magic itself may overpower their physical form.
Being inherently more powerful than mortals, demons have some more options when travelling between worlds.
Demons can inhabit a physical body in the Overworld. Doing so allows them to retain the ability to return to the Everglow, however if the body dies with the demon inside they will remain trapped in the body until it is destroyed, usually through cremation or decomposition; preservation is a known method of imprisoning harmful demons in the Overworld.
Possession occurs when a demon inhabits a mortal's spirit form during transcendence. It can happen by invitation or by force.
Through invitation a mortal transcends their spirit into the Everglow and allows the demon to take hold and return to the Overworld in their body. This is generally a form of sacrifice, however the demon can choose to leave by transcending back into the Everglow. This has been known to happen when great warriors from the Everglow are called upon in Crecia and they join the fight.
Through force malevolent spirits take hold of a transcended spirit and returns to the Overworld in their stead. This method is far more unstable than by invitation but that largely depends on the power of the demon and the mortal. Particularly dangerous demons have been known to be able to jump from host to host in the Overworld.
Demons using possession in the Overworld are not granted their full power that exists in the Everglow, nor are they inhibited with magic the way mortals are. They are made stronger and longer lasting than the mortal alone, however still vulnerable to death through warfare much the same.
Demons are also able to meditate in their own way into the Overworld in a similar fashion to mortals, without leaving a "physical form" behind in the Everglow. With this they have limited effects on the world, but have been known to possess mortals this way.
Demons can muster enough energy and magic to physically travel through midworlds into the Overworld. This is a long and painful process known as exodus, in which they travel through space and time into a completely different realm with different laws it abides by. Powerful, magical objects in the Everglow can also build up enough energy to be transported out of the Everglow and into midworlds; it is estimated that this is how the Starfall ended up in the Overworld.
Demons who undergo exodus completely leave the Everglow behind. In the Overworld they are immortal, powerful, and exist by different means than mortals; they cannot be killed in conventional ways, with some exception. However, they do live by the laws of the Overworld, meaning that things like time, gravity, and space naturally affect them in the same way. A result of this is that the same amount of magic in the Everglow has a significantly greater effect in the Overworld and vice versa, meaning that once a demon has entered the Overworld, they lack the magic and the means to obtain it to make a return trip. An exodus for a demon is permanent, and they can never return to the Everglow.