But this historical episode came with a caveat that had been unknown to Togame the Scehemer until she heard it straight from Shichika, in that Kazune had tossed the sword aside out of necessity – he couldn’t swing a sword to save his life.
The Kyotoryu – that school shrouded in darkness.
Had a skeleton in its closet.
What’s more, this complete and utter inability to wield a sword turned out to be hereditary, passed down to all the scions of the House of Yasuri – not even the genius Nanami Yasuri, much less her father, Mutsue Yasuri, could shed the fetters of this spell.

Katanagatari Omnibus 3, Page 249