If you cancel the order to cast mirage on a province it adds an item to your magic item treasury depending on what the target fortress type was.
Most significant is Crystal Citadel (dwarven hammer) but you can also get:
fire sword
ice sword
ice lance
halberd of might
smasher

Devs are being kikes about people reporting this so it's possibly getting fixed next patch. There's an open question about what other rituals might have bugs like this if this is how hackily and poorly put together their scripts are though.