Yami no Naka no Crown

Everyone in the world is born with two things: the ability to collect "Life's Light" and a "Light Vessel" in which to hold the collected "Life's Light". Most people use their vessels to assist them in doing various tasks but occasionally someone is born with a vessel that allows them to do battle with. The protagonist, Ruyu, stumbles upon the end of a battle only to find one survivor. Could the young boy she saved be connected to the king's men who entered the town with the intent of killing everyone?

(Source: Dan@PopulationGO)