Kuro Yori mo Kurai Ao

There used to be a God who preyed on the villagers. The best thing people could do is hide. Then one brave man took a sacred Taira sword and decided to give abomination a fight. Man was victorious, but he could only seal the rampaging God inside his body. For a long time his descendants inherited the seal.
But now the creature has accumulated enough power to turn the owner of the body into a monster committing and killing humans...

[From vndbreview]