Otherworlds

Fantasy Lands
The most well known of the magical worlds, the otherworlds are what people think of when they think of a magical dimension. Fairyland, Hell, Heaven, the Dragon Dimension, even Christmastown- these worlds are created from a stream of dreams and stories and nightmares and ideas (although many inhabitants of the otherworld consider it the other way around). This makes them very solid for magical dimensions- altering them involves going against massive magical power.

Unlike the more chaotic netherworlds, Otherworlds have consistent geography, factions and history. They're actual worlds, and can evolve past the initial conceptions that gave them birth. They're also the most common world, with thousands of them existing. Most dimensional travellers spend their time exploring the otherworlds.

Otherworldly Alliances
Unlike mundane worlds, the otherworld's highly magical nature and high numbers makes it much easier for them to interact. This allows for large scale interaction between otherworlds, creating organisations like the Seelie and Unseelie court, or the Crusade of Heaven. In most cases, these are benign, or at least as benign as politics gets. But in a few cases throughout history, empires have been formed, taking large swathes of the magical realm and even threatening the mundane realm.

In one notable case, the empress of an otherworldly empire fell into solipsism and turned the whole empire into the largest netherworld ever seen, a disaster that many otherworlds are still trying to recover from.