First introduced in the Majin Buu Saga, the Z-Sword is a heavenly sword, stuck in a rock on the Kai Planet. Through many eons, many individuals have tried to free it, but every attempt thereto, has failed, until the Saiyan Gohan managed to bust it out. It is said that whoever frees the sword, will receive tremendous power.
During a test session, Supreme Kai suggests to try and test its legendary status out on a massive-sized block of so-called "Katchin", the densest metal in the universe. Upon contact, the sword immediately shatters, which frees the Elder Kai from his eons of long captivity inside the sword. Presumably, this is what is meant with the person freeing the sword gaining massive power.
In the manga version of the series, the sword is known as the Zeta Sword.
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