For centuries, a special group of shamans are trained to find demons and reinforce the ้ญ้ (Konmon), a barrier that keeps demons from entering the human world. They slay down demons with their weapons as they create the Konmon with their voices, music and the ability to connect the fans' spirits with each other.
In the present times, this is being handled by two idol groups. A three member girl group named the Miko Gals and a seven member boy group named the Gekiss. When they collaborate, they go by the Hashira. Besides performing on stage and uplifting their fans, they secretly slay demons who try harming their fans.
You were attending one of their collaborative concerts, watching them perform on stage when you got pulled away by a demon. In such a crowd, nobody even noticed, but the idols did.
You found yourself in a dark alleyway as the demon covered your mouth so you wouldn't scream.