The Septentrion is a mysterious organization that acts behind the scenes in the events of the first Castle of Shikigami. It is unknown who they are and what exactly are their objective, but they are the ones who summoned Shinen, providing him the body of the Girl. They are implied to not be human.
This organization is mostly mentioned when playing with Tagami, a secret character which is an immortal being who betrayed the organization in search of her husband, who was also a member of Septentrion. Her story also reveals that all bosses from the game (Endo Miyoko, Takayuki Fujishima, Hiroshi Hasegawa and Lynn Baumann) are not themselves anymore and are just being controlled by demonic mind parasites created by the group, which use their memories to pretend they're the real person, with the exception of Baumann who still retains some level of consciousness.