Yasashii ryuu no koroshikata manga

Yasashii Ryuu no Koroshikata summary:

In this fantasy world, humans and phantoms live side by side. One represents Yang and the other represents Yin. A long time ago, when the world faced apocalypse, the King of the phantoms helped the Queen of the humans and made a promise to his beloved Queen that whenever her descendants faced a similar crisis, his descendants would come and help… Fast-forward to present time. An apocalypse is coming again due to the imbalance of Yin and Yang. A determined phantom King comes to the human world to fulfill his ancestor’s promise. He meets a beautiful Holy Knight by a random twist of fate, and so begins the adventure… Based on the same titled novel by TSUMORI, Tokio.