August Annihilation
Human-Forced demon bloodline.
Long ago, in a time before the modernization of man, there was a village that existed and was regarded as "The Village of Immortals". This was due to the fact that the leader at the time had found a use for demon's blood. By ingesting such a substance, the life expectancy for the inhabitants would nearly triple, however, not without its drawbacks. To have the appropriate result, one must maintain a calm and collected demeanor. If said demeanor or emotion be altered negatively, then the user is at risk of becoming a demon themselves.
Fast forward a few years later when the leader of the village had just...vanished, noone could figure out where he went or if he was even still alive. August and his older brother, Gennosuke, being the sons of the leader were now the ones being consider to take his place. Unfortunately, much to Gennosuke's dismay, August had been the one chosen to lead and with that the Village's Phoenix mark was branded into his lower back.
Of course, just a village that has partaken in something so taboo and unholy as ingesting the blood of the damned....there would be no happy ending for them. Willing or unwilling, all would eventually suffer. Gennosuke was left feeling betrayed, believing that he was better suited for the role. This causes him to lose his composure and without anyone knowing his change has begun and it is by his hand that the Village inevitably falls.
Alas, from the Village, August, Gennosuke, and their younger sister are the only Survivors.