What is the specialty of Nokogiri?
According to the Katanagatari wiki, the Nokogiri doesn't have any poison but has the ability to cure the effects of the poison from the other swords:


  A wooden sword that sharply contrasts its title as "King of Swords", it is in the possession of Zanki Kiguchi. Unlike the other Deviant Blades, it doesn't have any of the poison found in Kiki's blades. It is the purest blade with the ability to cure the effects of the poison the other swords have on their owners. A small floral mark and yellowish streak on the sword identifies it as Nokogiri.