Aria of The Darkest Night

Aria— a kingdom within the mountain's, residing itself away from the rest of the world. It's home to more than Thirty million people, going about their daily life's peacefully. Until one day, everything changes—
The warring fraction's of Alagácia, Yozora, and Veliciá continues without end. Day by day, minute after minute. When an emissary comes from Alagácia, asking the king for an alliance, the old king fears destruction over bloodshed.
Kuro, a young boy who lives his life branded as a street rat, becomes a royal bodyguard to the princess, Princess Yuuki. During this time of unexpected events, Kuro is forced to draw his sword against a familiar face, who is on the enemy fraction....