Realm

Realms are planes that exist parallel to reality. They exist in a completely separate sort of plane called realmspace, which is vastly unknown and ill-researched.

Mechanics
The laws of physics of realms is often substantially different to that of the megaverse. In some of them, physical space as a concept barely even exists, unless in specific areas of them.

They often can play an important role to the megaverse, as things such as the aether pool and the mana nexus appear to be the source of aether and thaums respectively. It's not exactly know if these were created by the megaverse or were seperate planes that attached onto it in some way.

They may be created artificially, but the process has been described as excruciatingly difficult and hard to control.