The 'Demon World' as I interpret it.

UUUHM. So basically it connects to all reality, including other worlds/times/universes, if you know how to use it. Most can only travel in the present in their own universe.

Four quarters, each seems to end when viewed from afar, but all extend into eternity.

In the universe, 'Demons'(who are just the natives of this realm, not actually anything 'religious') are the VAST majority, but places like Earth are more or less human havens. Some planets are set up like Earth, the demons going into hiding and allowing the humans to run rampant, but many worlds have humans and demons living side by side without disguises, and others yet have humans as slaves/gladiators/what have you. Some worlds don't even have any humans on them.

It is also basically the 'afterlife', but I'm not gonna go into detail just yet here.