Akuma-san to outa

Those who curse the world with their dying breath becomes demons. The ones who protect the country from these demons are the Children's Choir who have received divine blessings from the angels. Hymns sung by these children who are pure in body and soul are exceptionally beautiful. They are able to kill the demons with their pitch perfect harmony. As such, the demons are always looking for a chance to kill these children once night falls. However, there's one poor little boy who just can't seem to hit the perfect note...