Katanagatari

Synopsis

Back on the island, Nanami becomes annoyed with her brother’s slow progress and decides to set out and collect the swords herself. Along the way, she annihilates the Itezora clan along using methods much more violent and destructive than Togame and Shichika. She eventually comes to possess one of the 12 swords (Akutou Bita) and uses its power to strengthen her frail body by having it stabbed into her chest. Encountering his sister, Shichika fights his most deadly opponent during which Nanami asks him to kill her to spare her from the suffering that Bita brings with its power. After battling against Nanami’s all-seeing eyes and inhuman strength and Togame's life being threatened, Shichika is finally able to free his sister from Bita by killing her.

(Source: Wikipedia)