So we have to start with Nothing. Nothing (capitalized) is the physical substance of all "Unseeable magic". You know, anything "imaginary": dreams, memories, stories, music. Nothing looks a bit like molten gold in Nowhere (invisible anywhere else) and sort of floats aimlessly in and out of the mist. It cannot be destroyed, but it can be shifted into one of many forms, none of which can be turned back into pure golden Nothing (as far as anyone knows).
First, there is faded Nothing. When Nothing is forgotten, it shifts slowly from gold to silver, silver to gray, gray to white. By the time Nothing is white, it can never be retrieved ever again; the idea must be fully conceived again from scratch. Nowhere is a white expanse made of collected faded Nothing from all over the world. It's sort of misty, but not obscuring, just not quite revealing, if that makes sense.
Nothing can also be converted to Shadow, which is Nothing corrupted by Fear (we'll get back to that), if it enters the Darkness (we'll get back to that). You know, nightmares, mistakes, anything "imaginary" that haunts you for a long time. Shadow is shiny and black, except the shine is always red no matter how the room is lit.
On the edge of Nowhere is a dark expanse called the Darkness, technically part of Nowhere because they're on the same plane of existence, but basically the opposite. The Darkness is the home of Shadow and, similar to Nowhere, made up of faded Shadow. Shadow turns pure black as it is forgotten, then fades to gray and becomes part of the landscape. The Darkness is shockingly similar to Nowhere, but feeding on fear rather than inspiration.
And this brings us to the Kings of Nowhere and the Darkness. There are no people living in Nowhere, so why do they need Kings? Well, even the Kings aren't people as it turns out (plot twist).
So uh, before we move on, do you know those plasma balls they have at science museums and stuff? Well the Kings look kind of like giant plasma balls visually. The King of Nowhere has gold energy flicking around, and is made up of all the Nowhere that will never quite fade, even though the creators are dead. You know, things people will never quite forget, the "great minds". If you touch it, the Nothing within flicks to your hand (plasma balls) and you can get inspiration.
The King of the Darkness is mistaken for a lot of things, but (plot twist) turns out to be Fear. Its energy is red, and only the fearless may approach. Touching it will give you an overdose of Shadow, and it can lock you in a nightmare/hallucination until you go insane and/or turn evil (at least, that's how it looks from an outside perspective).
Okie, finally, how to get to Nowhere, because it doesn't quite exist. You have to know if its existence and be trying to get there (can't get there on accident, unlike Alteria), and you have to have a gateway. Gateways are odd and very rare, but they can be created: all you need is a lot of Nothing from a lot of people. One single object that contains it all. That can take you between Nowhere, Alteria, and Earth if you know how (or get REALLY lucky). You can get back home with the gateway, but a gateway will become corrupted and useless if it goes into the Darkness, as all the Nothing within turns to Shadow.