Darker than Black (2007)

Darker than Black is an anime series set in a world where mysterious gates have appeared, changing the sky and unleashing supernatural powers in some humans. These individuals, known as Contractors, use their newfound abilities for covert operations, while others hunt them down. The series follows Hei, a masked Contractor who works as an assassin while searching for his sister. As he delves deeper into the dangerous underworld of Contractors and secret organizations, he uncovers dark secrets and confronts his own troubled past.