Black edge

Everyone awakens at twelve. Kenshiro Ayase awakened at eleven. After years of imprisonment, Kenshiro receives an ability unlike anything he's ever heard of—Godslayer's Edge. But the weapon that appears in his hands is nothing more than a crude wooden sword. Weak. Useless. At least, that's what it seems. Because the moment Kenshiro takes his first life, the sword grows stronger. And so does he.