|“||"This sword was forged from the righteous energy within thee. It possesses the power to destroy this evil force."||”|
—Odin and Ra, Episode XXXVIII: The Birth of Evil, Part 2
BackgroundThe sword is the only power that can harm Aku. Its divine nature is capable of inflicting burning wounds to Aku which are very difficult for him to regenerate from. Enough hits with the sword essentially negates Aku's ability to regenerate his wounds, at which point further damage could presumably either kill him or seal him away again. Jack's sword can only be used for good, but in the hands of evil, it cannot harm an innocent. This was first demonstrated on one occasion where Aku stole the sword and attempted to kill Jack with it. Instead of impaling Jack, the sword simply struck Jack's chest without causing any harm at all.
However, this property appeared to apply only to biological lifeforms and not to robots, as Jack, when infected by Aku's essence to become Jack-Aku, either killed or grievously harmed an innocent robot with the sword, but appeared not to be able to kill the Lizard Monks whom he struck with it.
Very few characters other than Jack use the sword in the series, most notably the King of the Spartans in Episode XXV: Jack and the Spartans, after his own sword was broken and Jack threw him the sword to protect himself.
The sword has the power to cut through any mundane substance, given that there is enough force behind it (Jack was, for instance, unable to cut through the adamantium armor of the Ultra-Bots in episode XVIII without artificial strength enhancement).
Likewise, similar magically enhanced objects can also prove immune to being cut by the sword (for instance, the Scotsman's blade was imbued with magic runes and proved to be too resilient for Jack to destroy).
The sword itself, possibly due to its divine origin, seems to be essentially impervious to harm; it will simply rebound off of substances that it cannot harm instead of shattering upon impact, and can be used to block or deflect virtually anything without suffering damage. Unlike more mundane katana, which require extensive maintenance, Jack's sword appears not to require any sort of upkeep, though Jack was shown sharpening it on one occasion.
The sword's main purpose is to destroy Aku and defend the innocent; however, one very important aspect that Jack had failed to consider in his first battle with Aku, was that the sword is nothing without its original wielder.