Fushi no Sougishi

This is the story of the hero who must kill the princess. This is a world where the dead are sent to the final rest by the Immortal Undertakes, a race of long-lived beings who look human. Leon, a half-breed, is the youngest of them and is on his way to take his final test when his train falls off its tracks. He meets a human girl in the forest and though he doesn't know her, he somehow feels he must protect her... from the other Undertakers!

(Source: Mangamo)