The world of Samurai Jack is a wildly diverse place, filled with aliens as well as recognizable human cultures, monstrous creatures as well as recognizable earth animals and of course plenty of robots. But there’s one item that stands out as a symbol of the good and pure in all of its wielders. It’s the mystical Japanese katana Jack is always carrying with him. It is the one weapon capable of defeating Aku, the shapeshifting master of darkness and overlord of this future Earth.
It seems that this sword has some kind of magical properties, forged by the deities Odin, Ra and Vishnu along with the power of righteousness within its samurai lord wielder. It has the ability to only do good and in the hands of evil, it can not harm an innocent, which was demonstrated when Aku struck Jack with it and merely rendered him unconscious (Episode XXX: Jack and the Zombies). Even when used against Aku himself, the katana proves incapable of killing him as he is able to regenerate and reincarnate himself with relative ease (Episode XLIII: The Aku Infection).
The sword disappeared at some point after Season 4 ended. It may have been lost in the chaos of reviving the last time portal after Aku destroyed it. It could also have been taken by a character such as Blackbeard after he mutinied Aku aboard the Queen Anne’s Revenge, or Barbossa who claimed it along with his ship and crew.