YAMI NO NAKA NO CROWN

Yami no Naka no Crown summary:

Everyone in the world is born with two things: the ability to collect Life's Light and a Light Vessel in which to collect and hold it. 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. 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?