Hikaru ga Shinda Natsu

Synopsis:
Best friends Yoshiki and Hikaru have been stuck together like glue for most of their lives. Living in a small village and being the only children their age, the two boys find solace in each other's company. Their friendship seems picturesque at first glance, but Yoshiki has noticed a shift in Hikaru's demeanor ever since one fateful summer.
The "Hikaru" that sits beside him now is nothing like his friend. Despite his physical appearance and memories remaining exactly the same, Yoshiki knows something is amiss and that the Hikaru he once knew is gone. When he brings this up, Hikaru partially transforms into an otherworldly being, threatening to kill Yoshiki if he cannot accept it—while also professing its love for him.
Wracked with grief and unable to let go of his best friend, Yoshiki struggles to move on with his life as he finds himself getting closer to the new Hikaru. At the same time, strange occurrences begin to plague their village, bringing unsettling chaos.