Shinmyoumaru Sukuna is an inchling and a descendant of Issun-boushi. During a past incident, she decided to use the magic of the Miracle Mallet, a sacred treasure of the inchlings that had been sealed away, to create a paradise where the weak wouldn't be discarded and abandoned...

Reimu Hakurei is the Hakurei shrine maiden. During a past incident, her purification rod started moving all on its own. Sensing it had something to do with an incident, she grabbed her purification rod and flew off toward the Inverted Castle that had appeared in the sky! Incidentally, what Reimu calls her purification rod is actually an oonusa.

In the original tale, Issun-boushi acquired the Miracle Mallet after defeating an oni. It's said that he grew taller upon swinging the mallet. Not only that, but he also ended up falling in love with a princess. The tale of Issun-boushi has been told for generations all across Japan.

"I wish to build a castle of great splendor and rule over the people!" exclaimed Issun-boushi's descendant as they swung the forbidden Miracle Mallet. The mallet is an oni item, and thus using it incurs a cost proportional to the wish you make. And if your wish is too grand, you'll be forced to pay the price before it comes true...

The inchlings were confined to the world of the oni, where none ever spoke of what happened again. That may explain why Shinmyoumaru didn't know about the Miracle Mallet until she met a certain youkai. The amanojaku recounted to her a false history―one which tricked Shinmyoumaru into rising up alongside the other weak inhabitants of Gensokyo.