Kyoka is another tool of Aoi Uchiha just like Sakanade, but out of the 2 this is the strongest. It looks like a regular sword, but its not, this weapon he keeps in his very long hair, you cannot tell the weapon is there because he puts a transformation jutsu on the sword, so it looks like strands of hair. This is also a weapon he really doesn't use unless he has to, also it takes up a lot of chakra when he uses this sword.
When Aoi took it to a blacksmith, the smith told Aoi that the weapon wasn't made from any common material like steel. The smith said it was most likely made centuries ago, using an unknown material, then the smith told him the the power that the sword had it could of possibly been a tool of the Sage of Six Paths, or his decedents.
Its special function lets Aoi basically mess with a persons 5 senses. When the sword is released by Aoi funneling his chakra into it and waving it in front of the person, if you look at the sword when Aoi waves it, you are in the function of it already, with this your senses get totally messed up. It puts you in a hypnotic type of state, though, so you may attack Aoi, but in reality you are attacking teammate or tree. He can use this sword and Sakanade to make an ultimate pair of dual swords.