Thousands of years ago, aiming at destroying each other, angels and demons broke out a long-lasting war, which left the human world almost collapse. In the end, the two parties reached an agreement called 'Millennium Covenant' to withdraw the human world.

It seemed that the 'Millennium Covenant' the war had come to an end but unseen crisis kept dwelling. One day, the Temple of Wisdom was finally destroyed and the Book of Wisdom disappeared. A few years later, an ability called 'Anthem' began to appear in the world. No matter a sacred knight of the Holy See, an indomitable Rebel , a strange demon or a mysterious demon hunter, will be inspired by the 'Psalm' and become a hero to fight for you!

And this is just the beginning of our story...