Claymore

Synopsis

Clare receives a black card from mysterious man who seems to be working with the organization she works for. Raki witnesses as Clare confronts another Claymore named Elena. Clare explains that when Claymores use too much Yoma power, they eventually succumb to the influence of their Yoma half. Once this occurs, Claymores, knowing they will have to die, often make the choice of dying with human hearts, so as they feel their humanity beginning to slip away they send a black card bearing their individualized insignia to the Claymore that they want to be slain by. Clare's closest friend, Elena, was on the verge of succumbing to her demonic nature and becoming a Yoma, a process called 'awakening'. Despite Raki's protests, Clare slays Elena, granting her the solace of dying as a human.

(Source: Wikipedia)