Mr. Drop yo girl

A dark clan is decimated by a righteous hero. The most righteous and empowering hero, for that matter.
Due to some unknown phenomenon, be it guilt or pity, a child is spared from the hero's cleansing.
And now that child seeks vengeance for his fallen brethren, he sets off on a journey to end the righteous hero's tale, unbothered about morales and goodwill.
The boy only seeks honor from his fallen, and escapism from himself.
In a time of peace in a world of overruling light, a dark anomalies crashes forth, ready to pierce through said light as it's final avenger.
The hero's righteousness will be his brutal hamartia.
Either that or the boy's blindness will lead him far down a vicious path and he will meet with a foul end.

A story of conquering foes, gulping down betrayel and losses whilst despising the light.
It all starts and ends with a steel blade.