The alternate universe on the other side of the Forbbiden Mirror. It is very different from our universe, because it operates by different "laws".
The "laws" of this universe are very unusual. While it does have diffined space and time as well as gravity like ours, there are several major differences, the most prominte being that any living being in the universe is put into a coma like state, their mind however is projected to a overall view of the Realms and in some cases it was believed to be a prison ceated by the First Dark Empire. After being absorbed by Lunatic Pandora it is possible that if the Pandora were to be considered a Castle then the "Forbidden Mirror" and its "Other Universe" would be its dungeon.
- Light Incarnate (also known as "Other Kingdom Hearts", "Other Universe's Kingdom Hearts" or "God of the Other Universe")
The "Other Universe" is believed to have been a prison for usage on Keyblade Masters of the Realm of Light, the "Other Universe" was therefore created as an infinite prison of sorts where the mind of the one trapped within it would be forced to watch the events happening in the Realms, this effect would either drive a Keyblade Wielder insane or to use the Darkness or both.