YukiYuna is a Hero

From the introduction: It is 300 years into the Divine Era. A virus has wiped out most of humanity. The survivors living on the Japanese island of Shikoku, protected by the god Shinju-Sama. No one is allowed to leave the barrier, and no one has arrived from the mainland. ...Until, inexplicably, gigantic creatures called the Vertex appear from beyond, with the intent to kill Shinju-Sama and with it, humanity.