Nasu and Takeuichi's first collaborative work and the second work in the Nasuverse (third if you count the unreleased Mahoyo draft)
The series focuses on Ryougi Shiki and Kokutou Mikiya and at first is somewhat of an episodic series dealing with the supernatural cases they get involved in working under Aozaki Touko and her agency - The Hollow Shrine. Although, from as early as the second story, a backstory and overarching mystery surrounding the events surrounding the 1995 serial murder case in Mifune City and Shiki's involvement in it.
h