Touhou Suimusou: Immaterial and Missing Power

Gensokyo has settled down after the Spring Snow Incident that occurred in Perfect Cherry Blossom. Reimu Hakurei and the others are currently having a feast every three days at the Hakurei shrine. During these feasts, a strange mist appears. The playable character decides to get the cause of the never-ending feast schedule and the mist. This incident was later called the "Night Parade of One Hundred Oni Every Four Days" by Aya Shameimaru in Bohemian Archive in Japanese Red.

The perpetrator of the incident is an oni, said to have disappeared from Gensokyo, named Suika Ibuki. She thought that it was a shame that the flower-viewing season had been cut short due to the very long winter. She spread herself out in the form of a mist all throughout Gensokyo, and "gathered" the hearts of both humans and youkai to cause them to have a feast every few days.

Because the art of oni extermination has been lost to Gensokyo, the girls could not eliminate her, but the mist disappeared afterwards and feasts started to happen less frequently.

Résumé

Touhou Suimusou: Immaterial and Missing Power is the 7.5th installment of the Touhou Project by Team Shanghai Alice's sole member ZUN along with conjunction with Twilight Frontier. It is called "7.5" because the events chronologically take place between Perfect Cherry Blossom (TH07) and Imperishable Night (TH08). It was developed alongside Imperishable Night and released at the following Comiket 67.