Senketsu no Elf 

What was left for the boy, was just the girl. And what the girl chose, was just the boy. Human siblings living in a remote village, elven siblings living deep in the forest. The peace of theirs, who should not have been friends, should have continued forever. However, when the elves suddenly started an invasion on the human world, the feelings of the four were torn apart and they took different paths. Eventually, the human boy Imina and the elf girl Ellis stood on the battlefield. The sword he holds is crimson. The same color that had flowed on it, and will flow on it from then onwards... The sharp, blood-colored dark fantasy begins here!