Shouki no Gas Masquerade

When the Miasma appeared, it devastated Japan. Killing humans and transforming animals into vicious monsters, the Miasma also became a limitless energy source for those who survived. Kenga Yuuki wants to become a monster hunter like Ryou, the hero who saved him when he was younger, but the obstacles he'll face in this quest are far greater than he could ever have imagined!

(Source: Mangamo)