Keeper of the Horses is a powerful spell in Campione!
Years ago, in order to defeat and harness the power of the Heretic God, Monkey King Sun Wukong, an unnamed mage created a spell of immense power. Possibly the most powerful binding spell to ever be cast by a mortal. This spell petrified Sun Wukong's body, all but his head, sealed his divinity, and created a prison for him within the Netherworld.
However, the prison was not meant to be inescapable. It required three things; a Hime-Miko with the power of Disaster Purification, the treasure sword Zanryuuto, and a snake or dragon Heretic God. With these three things, Sun Wukong would be freed for a short time to battle his enemy, the snake or dragon Heretic God, and afterwards would (eventually) be forced to return to his prison.