Netherworlds

Subjective Worlds
In the world of magic, reality is dependent on will. Strong enough will can override physics, geography and other wills. Pushed to its extreme, this creates the netherworlds

A netherworld is a world completely dominated by a single mind, existing solely as a reflection of that person's psyche. Anything that enters is slowly changed to fit that worldview. While there is nothing strictly stopping a benevolent netherworld from forming, the kinds of minds interested in complete domination of a worldtend not to be nice people, thus explaining the names "netherworlds" and the sobriquet "dark lord" for those people.

Most netherworlds have a strong urge to consume other worlds, breaking into netherworlds, otherworlds or wild magical areas and subsuming them into their own worldview in a process half pitched battle and half ideological debate. in some cases, huge dimensions formed of cobbled together netherworlds have risen and fallen.

Some netherworlds have even attacked Shadow Dimensions or Probability Space. It is worrying that would succeed, and more worrying as to the implications that it may one day succeed on Earth Prime.

Claiming Netherworlds
Entering a netherworld is a feat of wills. While these worlds contain very unique resources, staying there too long turns you into a minion of the Dark Lord. Strong will can protect you, but may cause the dark lord to take active steps against you.

While the Dark Lords are extremely powerful in their home domain, they're not omnipotent (who has complete control over their own mind, after all?). Killing a dark lord collapses the netherworld back into random magic. However, it is possible to claim control of a netherworld from the dark lord. If you do this? Well done! you own a dimensional reflection of your innermost psyche!

As a sidenote, it is possible to accidently create a netherworld. A person in raw magical areas can, through intense emotion, accidently lock themselves in a reflection of their own mind. These places tend to be unpleasant, and many magical heroes dedicate themselves to freeing people from them.