Believer!

"Once in a thousand years, the door to a parallel world opens, bringing strange beings into our world. However, when this time comes and the world falls into chaos, the legendary miko* and her guardians will be our saviours--"

Following the prophecy that has been passed down for several thousand years in history, the door to a parallel world suddenly opens, and strange beings appear before a group of high-school students who had been living peaceful lives.
These students accept their fate — to become the miko and her guardians — and decide to fight in order to save their world.