Yōma (妖魔) is a type of parasitic demon born from bloodshed between shinobi. If the blood within a shinobi barrier flows freely enough, it can break through and emerge into the human world. It is the duty of Kagura to face off against yōma on the front lines.
Under shinobi law, only those who have graduated from school are allowed to be told about their existence. However, there are circumstances, such as the case with Miyabi and Imu, where they encounter them before that time.
All large-scale calamities suffered throughout the world originate from an ancient, yet still-living, yōma known as "Shin". It's the heart; the soul. As long as the ancient yōma remains undefeated, major disasters will forever continue to plague the population. These disasters will mount and intensify over time, claiming countless victims every step of the way.
In order to defeat Shin, it must first be pulled into the shinobi's own time. The only way to do that is with the shinobi barrier. Blood spilled within it is the most effective bait to lure yōma out from hiding.