“…And the inhabitants of the Dwellings pleaded: rivers of our blood and other useful fluids are flowing in our homes, spilled by the arrogant hero-invaders!
And the Gods heard these prayers.
And they gifted each Dwelling with a personal Summoning Crystal.
And near these indestructible Crystals, in each battle, reinforcements began to appear, to aid the locals, and to bring utter doom to those who came…”
Quoted from "Summoning Crystals: History of Appearance, Structure, and Principles of Operation," vol. 2, pp. 24-25.