From the back of the book:
When the red moon rises, the gate to a strange world opens
The red moon, that is the true moon
The book "Red Moon" is a sourcebook containing world background settings and additional data for "Night Wizard! The 2nd Edition". Presently, Far The Earth is slowly heading towards destruction. Thanks to the fight of the wizards, the world has been preserved, but the number of people noticing that there is something else on the other side of normalcy is increasing. In a world growing more dangerous, the wizards are protecting the illusion of normalcy.
The world kekkai has lost its former solidity, and has now become like an unstable, vibrating bubble. Probably because of this, the world has granted the wizards a new power. That is the new kaguya, the growing kaguya. It changes the world into a shape fitting itself, putting the world under its control.