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The Celestial Plane is a extra-dimensional realm that the Divines live in.
Not much is known about how the celestial plane came to be, except that it was created by "The Creator" along with the divines. The appearance of the celestial plane is briefly described in Chapter 25 :
"Memeta teleported to yet another one of the many abandoned rooms in the celestial plane. She didn’t really know what function each of the small 10m x 5m rooms served, whatever they were for didn’t matter anymore. All they were to her was an annoyance. Crypto could be hiding in any one of them. This room was like all the other hundreds of rooms she had searched, it was embedded into a bank of the cloud-like substance that made up a large amount of the celestial plane."