Eden its only one of the four planes that form the Cosmos, in addition to Eden there are the dimensional planes of Genesis and Bane and the non-dimensional plane of Limbo.
They all share time but not space. Every plane except the Limbo has its Guardian Entities, if one of the Guardian Entities of the plane kills a foreign entity to this its soul will never be able to return to the world in question. For example, because Luane was a Guardian Entity of Eden because of her dragon soul and she destroyed Azzoïl body in Eden, because of that, Azzoïl will never be able to step on our world again.
The Limbo (Spiritual Realm)
In this plane there is no space, as there is only time. This is where most souls go to later reincarnate in Genesis or Eden as appropriate. Each type of soul has its own cycle and only sapient beings have souls. An species soul cannot reincarnate in another specie. There are exceptions, for example elisians and human share soul type with humans and assmar can sometimes bear the soul of a dragon (Wich makes them very powerful and dragon-like, due to their unusual soul). Even in such cases, the reincarnation cannot change the nature of a soul.
It is impossible for the living beings to stay on this plane of existance as corporal beings cannot exist outside space itself. If a demon managed to redeem itself enough, it couldl end here, but only Dalyn demons have done such a thing.
Limbo lacks Guardian Entities.
Bane (The Countless Infernal Realms)
On this plane, highly reminiscent of the Hakru, is where demons live, if the soul of a non-demon has been VERY vile in life its attraced here, as well as normal demon souls, making the demon able to create more of them.
It is made up of hundreds of realms, each with its own morphology and traits. Like the other planes, time goes by normally. Each world has its Demon God that controls and rules it. Access from an Infernal Realm to Eden can be costly depending on the cosmological distance between Eden an such Realm, promoting certain aspects in the Material Plane can bring the cosmological distance closer, making the Demon God able to influence the world better, and when the cosmological distance is close enough, enter with its armies. The more powerful an entity is, the more difficult it is to enter Eden.
The Demon Gods are the Guardian Entities of Bane, but only on what concerns their particular Realm.
Eden (Mortal Realm)
This is where earthly beings live and die, then they go to Limbo and then reincarnate again. It has only has one known world, but it is possible that there are more worlds in other solar systems.
Dragons are the Guardian Entities of Eden. Beings with dragon soul like half-dragons and a few assmars also count because the Guardian status is a matter of soul, not biology.
Genesis (Elemental Realms)
Genesis is divided into tens of pure elemental and mixed elemental realms, each with its own morphology and traits, based on the elements that compose it. Like all planes, time passes normally.
The realms of fire are planets of fire or hot rock, the realms of water are planets made of water, the realms of air are unbreakable bubbles of planetary size that contain air in their interior, the realms of earth are rocky planets. Mixed planes can combine feaures or combine them.
The Elemental Lords are their Guardian Entities and are inhabited by creatures such as the Jin and the Elementals. Very little is known of such entities, as they don't visit Eden often.
The Great Cosmic War
The story of the Cosmos is not about good and evil, but a much primal struggle: Dara vs Hakru. The Dara is the world as we know it and also the energies of creation while the Hakru is the previous state of the universe, literally void in the demon tongue.
The return of the Hakru the demons want, implies the destruction of the Dara, the destruction of the creation that ended the void. But while demons and the Hakru are the absolute evil from our perspective, their opposite, the Dara and its inhabitants, are not the absolute good. Dara is about the universe as we now it existing, and the universe its not by itself good or evil, it just exists. In short, the cosmic war is about existance against void.