Hi Raccoonoo! Thank you for enjoying the book! A Cathedral is a mental subspace that an exorcist could project into the real world through a High Blasphemy gained by bonding with the virtue Temperance, a version of temperance made in a homebrew by a friend of mine! However long since writing it, their homebrew project the Icarus Doctrine never came to fruition as a page on Itch and is also currently unavailable as an official source for the forseeable future. If you are interested in it however, ask around the unofficial CAIN server! There are a few copies going around.