During Dragon Ball Z's Buu Saga, the series took a break from Fusions and five-minute screaming power-ups to introduce its own version of Excalibur. The Z Sword is an ancient weapon embedded in a plateau on the Sacred World of the Kai, where it has been stuck for over 1,000 years. The legend behind this sword is that only the chosen one can lift it, and when they do, they will gain incredible power.
The Z Sword has appeared or has been mentioned in both Dragon Ball Z and the Dragon Ball Super manga. Gohan and Future Trunks both trained with it during their mentorship under the Supreme Kai. The mystical weapon also served as Old Kai's prison, where he was trapped for over 1,000 years as punishment for the crime of annoying Lord Beerus. Here's what made it special throughout the series, despite becoming something of a running gag.
Gohan and the Z Sword
After Gohan gets rag-dolled by Majin Buu, he is taken to the world of the Kai in hopes that he will be able to lift the Z Sword and use its incredible power to defeat the pink villain. Considering the ridiculous feats of strength most characters in Dragon Ball are capable of, the Z Sword is impressively heavy as Gohan needs to go Super Saiyan before pulling it from the stone. Shin and Kibito are astonished he's able to remove it, leading them to believe they have found their chosen one.
However, the reverence of the Z Sword is quickly traded in for some father and son shenanigans once Goku arrives. The Kai are left speechless when Goku can also lift the sword, giving the impression the mystical weapon might not be that special, but rather the Kai are just weak. In the first of two instances during the Buu Saga in which Goku and Gohan playing catch severely backfires, Goku tosses a hunk of the densest metal in the universe at Gohan to test the sword's strength -- which unceremoniously snaps the legendary sword.
A bemused Goku and Gohan -- alongside a mortified Kibito and Shin -- are greeted by Old Kai, who was able to escape his millennium-long imprisonment inside the sword now that the Saiyans reduced it to "the Z Dagger." Every Flying Nimbus has a silver lining, as Old Kai quickly gets to work performing a long ritual that unlocks Gohan's hidden potential. In a roundabout way, the prophecy regarding the Z Sword was true, as the person who freed it from the stone did indeed receive a major boost in power as a result.
Future Trunks and the Z Sword
The Dragon Ball Super manga reveals how the Buu Saga played out very differently in Future Trunks' timeline -- in that it never even occurred. Future Supreme Kai had wisely enlisted and debriefed Trunks regarding Babidi's plan to revive Majin Buu before the evil wizard appeared. Trunks was brought to the Sacred World of the Kai and trained using the Z Sword, and he did indeed become much stronger by doing so.
Trunks took the Z Sword into battle against Babidi and Dabura where he was successful in destroying them both and preventing Buu's revival. The only casualty of this minor scuffle -- compared to what happened in the main timeline anyway -- was the Z Sword itself. Dabura's spit has the power to turn anything to stone, but Trunks also dropped it, and the poor thing shattered. Though it remained unknown to anyone in this timeline, the result meant that Old Kai's life was lost forever.
The legacy of the Z Sword during DBZ and Super is a masterclass in subverting expectations. Particularly in the Buu Saga, misdirection is the name of the game. The world was never going to be saved with plot devices like Super Saiyan 3, the Fusion Dance, the Z Sword or Potara Earrings because, in the end, all roads lead back to Goku. In true Dragon Ball fashion, the Z Sword fulfilled its narrative purpose and then became a punchline, just like Yamcha.