In Another Life, We Only Had Each Other

Eren and Zeke after their deaths were reincarnated into a world with demons instead of titans like titans they feasted on humanity, Zeke having a recollection of his past life unlike his younger brother sought them to become the strongest, when Eren is badly injured by a swordsman he does the only thing that would save his little brother's life and his, he became a demon. KNY x SNK