Madoka Magica: The Different Story Volume 3 (Manga)

 When she fails to save Sayaka, Mami is overcome by despair and loneliness.  With the fate of every Magical Girl made clear before her, Mami chooses to embrace death rather than fight on.  As Mami sets out to deliver herself into the hands of the witch, Kyoko rushes to confront her former partner.  Mami's soul gem steadily darkens - soon her doom will be certain.  Can Kyouko give Mami a reason to fight on before her gem turns jet-black?