“Either way, her time was up. If you hadn’t been the one to kill her, her illness would have killed her. Think of it that way, Shichika.”
“If I hadn’t taken Akuto the Eel from her… would sis still be alive?”
“You’re mistaken.”
Togame raised the dagger in her hand for him to see – as if to warn him.
“Nanami was killed by Akuto the Eel. It’s just like you said before. Swords can’t handle other swords. This thing isn’t called the Eel for show – it sucked the life right out of her.”
“I guess Mt. Shirei, where she found the sword – was known as a hangout for monsters.”
“Not monsters, so much as ghosts. I’m not saying I believe in all that hocus-pocus… but this sword was the only thing keeping that moribund mountain alive. It was bound to die. You can’t force nature.”

Katanagatari Omnibus 3, Page 99