Mouryou no Nie (2012)

Hundreds of years ago, a big war between humans and monsters happened. Humans somehow won the war and sealed up the monsters deep in the mountains.... Hayato, Chihiro and Suzuna work hard to eliminate and seal up evil monsters every day. They are actually monsters, but they just want to coexist with humans. One day, they hear that the monsters sealed up hundreds of years ago are about to come out... To stop it, they decide to visit the place.