D.D.D.

Since the formation of the "Black Knight Squadron," the kingdom of Galronia has expanded its military presence rapidly. The Black Knights were heralded as heroes, protecting the country from its enemies, but they are actually demon-possessed warriors. Not knowing this, young Julius sets out to join the ranks of the Black Knight. After being led to a battle royale against the other applicants, a strange mist fills the room they were all led into. Next thing he knows, he wakes up with a demon possessing his left hand... and the other applicants have turned into hideous monsters. With the help of the demon in his left hand, Julius must now save his sister and younger brother from the grasp of the Black Knight.

(Source: MangaHelpers)