Read order
Touhou - Yagokoro Syndrome
Touhou - Time of the full moon
Touhou - Undying

The story is related and continual from Touhou - Time of the full moon. After Kaguya and Eirin has built Eintei and settle down. Kaguya decide to take a walk. Someone attack her suddenly. Yes Kaguya is immortal, there is no way she will die. But is it really that simple?

Note:
I believe the attacker name isn't mentioned.
Her name is Fujiwara Mokou. She is Kaguya's bitter enemies