Honoguraki Toki no Hate Yori

It was the forbidden summer.

Working as a detective's assistant, Gojou Kouichi returns to his hometown island at the request of a client searching for a friend who has gone missing. His little sister who came to welcome him at the harbor, his parents’ guesthouse and the state of the town all appear to be unchanged. However, rumours of a strange creature are heard within the island, and one night a doubtful Kouichi is assaulted by a monster. Inui Komako, a lone girl who seeks to destroy it unbeknownst to the villagers, saves him from the clutches of death with blade in hand. Kouichi was certain that he saw it. He saw the monster appearing from obscurity, but also the realm spreading out from the ends of darkness. It was a sight that humans must never lay eyes on.

The forbidden domain.

Translated from Official Site; adjusted with personal description