Kono Shima ni wa Midara de Jaaku na Mono ga Sumu

The college student Kajiura Tarou, together with his research group including the mysterious and beautiful senior, Yoruto Haru, visits a distant solitary island, Ikishima. However, in this island which is also the hometown of Yoruto Haru, there is an ancient custom wherein outsiders should never be allowed to step foot into it during the period of the festival....... The inhabitants who have undergone a sudden change now closes in on Kajiura......!