Shingeki no Kyojin

In a world ravaged by man-eating titans for over a century, humanity's rare survivors have no other choice but to barricade themselves in a fortress-city to survive. Humans haven't seen any titans near their city for over a hundred years. Eren, the hero of this manga, and his adoptive sister Mikasa will nonetheless witness their mother's death when an even larger titan than the previous ones appears to destroy the wall, the Colossal Titan. Eren then decides to get his revenge and kill all titans by joining the Survey Corps, humanity's elite soldier section.