Space Creation Spell is an archaeomancy spell described in Monster Girl Encyclopedia World Guide III: Sabbath Grimoire.
Guide Book Entry
A spell for creating a new space called a “spirit realm”. This spell creates new “worlds”, which was originally the work of the gods, so to speak. It's the highest level spell, classified as the most advanced spell in existence, and it requires the most mana. Of the large scale spirit realms created using this spell, “Pandemonium”, created by the fallen god, and “Wonderland”, created by one of the daughters of the mamono lord, are among the most famous.
An archmage would have to expend a vastly enormous sum of mana simply to create a small room where nothing exists. Creating a space the size of a kingdom and then creating land, nature, and buildings there would absolutely require godlike power. But since it has become possible to infinitely accumulate power by having sex with one's spouse in the present era, there are a fair number of monster and incubi casters with power to the degree of a lesser divinity, and it has now become easy to create small scale spirit realms using the overflowing mana that is continuously generated in mamono realms.
For that reason, things have gotten to the point where it is no longer rare for a powerful monster to prepare a room in a spirit realm to use as a bedroom for living with her husband, or even to have her own small mamono realm in a spirit realm, as if owning a villa in a summer resort. Our Sabbaths also possess numerous branches, test sites, and venues for black mass in spirit realms.
Spirit realms are created in different dimensions than the current world, but the dimensions intimately overlap, and it is supposed that they are somehow connected to it. Such overlapping places can serve as entrances or exits to spirit realms. In some cases, doors or gates, etc. are artificially established so that the entrances are easily recognizable. But in the case of Wonderland, there are lots of entrances that were randomly generated. Mere hollow trees and holes in the ground, etc. can be connected to Wonderland, and there are even cases such as when people walking through forests unwittingly wind up in Wonderland. Furthermore, even “Pandemonium”, which supposedly has no entrances or exits, and can only be accessed via a spell cast by adherents of the fallen god, does have more than a few places that overlap with the current world, and people do randomly stray into it on extremely rare occasions. Also, when spirit realms nearly overlap with places in the current world in such a manner, they sometimes exert influence. It is supposed that humans who live in lands that nearly overlap with Pandemonium are highly likely to awaken to the fallen god's faith. As for the closest spirit realms, in some cases there are near identical spirit realms created at the “backside” of structures, villages, etc. that exist in the current world. The venues where our Sabbaths secretly hold black mass in human villages are also mostly created in places such as the “backsides” of the chief god's churches.
Even after creation, mana is required to maintain spirit realms, but since all living things produce mana, spirit realms can be maintained as long as life is active there. If life goes extinct in a spirit realm, then it will gradually contract and eventually disappear. Conversely, as long as life exists in a spirit realm, nothing can extinguish its existence. What's more, if human and monster married couples live there, then sex will result in the production of vast amounts of mana, enough for a surplus even after paying the spirit realm's maintenance cost. That's why as the number of couples inhabiting a spirit realm increases, a spirit realm will naturally expand larger.
“This is our bedroom, where you and I will make love... This is the place to which you will return. You thought you opened the door to your own room...? No, no matter where you try to go, you'll always end up returning here... and coming back to me...♥”
~a lilim, a daughter of the mamono lord~