The Realm is a world that was created by Noel Vermillion through her own will to find the strength needed to save Tsubaki Yayoi. Its appearance is similar to select locations within the 13th Hierarchical City of Kagutsuchi.
When Tsubaki was being controlled by Izanami using the Mind Eater, Noel wanted to find the power within herself that could be used to save her friend. Through her own will to seek power, she managed to create the Realm, a space free from Observation and Phenomenon Intervention. From her room in the Novus Orbis Librarium branch within the 6th Hierarchical City of Yabiko, Noel unconsciously entered the space, and Rachel Alucard followed; within Noel awoke, finding herself in her old NOL uniform.
Inside the newly awakened space, Rachel helped guide Noel to realize her resolve; Noel was able to accept her power as Mu-12 and gained the ability to access the Lux Sanctus: Murakumo afterwards. The two had their plans foiled when Hazama and Yuuki Terumi forced their way within the Realm, using the tiny tear that was created by Rachel's entrance. They made Noel battle against the real, and recently resurrected, Nu-13. Unable to finish the battle, Noel and Rachel exited it. Thanks to Rachel's efforts, Noel was able to find the power to save Tsubaki, but the Realm has not been seen since.
The Realm was created during a time when both the Amaterasu Unit and the Takamagahara System were utilizing Phenomenon Intervention against one another. As well as this, Kokonoe had her own intervention in the NOL branch where the space was made. Within the space, one will feel like they are in a dream, but they are able to feel pain and all memories are crystal clear.
BlazBlue: Central Fiction Library Entry
A world Noel created when she gained the 'will for power.' It feels like one is in a dream, but one is able to feel pain, and one's memories are crystal clear.