The Summer Hikaru Died, Vol. 4 - Bog

Two boys grew up together in a certain village--Yoshiki and Hikaru. One day, Yoshiki became sure that Hikaru was no longer himself. He resolved to accept the new "Hikaru," whatever happened to his friend. But now he's decided to research the history of "Nounuki-sama" and Kubitachi to understand what this creature truly is. The situation evolves unpredictably, outside the boys' control. Who will make the next move, and what will they find?