Suo – Ganryu Island.
One of the Two Great Holy Sites for swordsmen, where over two hundred years ago, a man bearing a greatsword fought another bearing two. And alter on, it was the site of yet another epic battle – in which the Strongest fought the Swordless, carving a new legend into the history of the island.
And yet, the details of that battle – how the Strongest vied against the Swordless, and how the Swordless overpowered the Strongest, remain unknown to all.
But the fact that Ganryu Island shrunk to less than half its size as a result of their duel is testament to the ferocity of the battle.

Katanagatari Omnibus 4, Page 315