Manila By Night
A Demense is an impossible place. It is a place where the Supernal World leaks into the Fallen World, drawn down by the power of Sympathy with a mage’s soul stone. Within the bounds of the Demense, magic is free from the bonds of Paradox, the clutch of the Abyss beaten back by the soul stone’s power. Within a demense, a mage might work incredible wonders impossible out in the Fallen World.
All that power comes at a price of course. To create a Demense, a mage must willingly distill a part of his soul down into a material object, a soul stone (a misnomer, as nearly anything can be designed to carry such a shard, from crystals to animal fetishes to architecture and art objects). Doing so hinders your own magical growth, in return for temporal power. It also comes with grave risk. If a rival were to seize a Soul Stone, they would gain a staggering amount of sympathetic power over the mage.
It is said that the Seers of the Throne are particularly skilled in the crafting of Demenses, weaving strange arcane works called Prelacies into their soul stones to create locations particularly strong in a kind of magic, while being actively hostile to the workings of Pentacle mages.