A single, limitless wish, from the strongest source of magic left in that world: the Holy Grail, some called it. Wizards submitted their minds and souls into it for the possibility of passing the Preliminaries and summoning forth ‘Servants’ to fight for them—powerful figures from history, myth, or legend that were made manifest and bound to them, in a fashion.

[The ones that knew what they were getting into were desperate and and dangerous and very, very cunning. With a prize like that on the line, you had to be, Hakuno supposes.]

That’s where it gets tricky. It was in that world long before the Mirrorbound showed up, and we’re unraveling what we can, but so far the most coherent lead is ‘the goddesses of the twin moons unleashed it long, long ago to punish the fae for teaching humanity magic.’ So. There’s still quite a lot left to uncover.