The Dragon Ball Z series left one subtle tease unresolved, but the Z Sword mystery was resolved in Dragon Ball Super, with Beerus at the center.
Among the new transformations and inter-universe tournaments, Dragon Ball Super found time to clear up an old mystery about the Z Sword. Devised by the original creator, Akira Toriyama, Dragon Ball Super follows Goku and friends a few years after the defeat of Buu. Along with the 2013 Battle of Gods movie, the sequel series introduced new divine beings called Gods of Destruction, whose galactic importance rivaled even the Supreme Kai. Where the Kais are considered “creators” in the universe, Gods of Destruction, as their name implies, are required to maintain a healthy cosmic balance by blowing stuff up, which is where Lord Beerus comes in. After a 39-year nap, Beerus sought out the Super Saiyan God, leading him to the last refuge of the Saiyans, planet Earth. Thanks to humanity’s delicious food, he’s been a regular visitor ever since.
During the Majin Buu saga, the Supreme Kai took Gohan to the Sacred World of the Kai and introduced him to the legendary Z Sword, which was trapped in an Arthurian-esque stone. Shin stated that if Gohan could free the embedded weapon, he would gain power tremendous enough to defeat the troublesome Buu. Although the half-Saiyan did manage to liberate the sword, its reputation turned out to be somewhat inflated, and the blade shattered after Gohan and Goku put it to the test. Unbeknownst to all, the Z Sword had actually been the container of an old Supreme Kai. The newly-freed pervert had apparently been tricked and sealed away fifteen generations before by a terrible villain who possessed awesome powers but wasn’t as bad as Buu.
It’s unusual for Dragon Ball to allude towards a new villain only for the tease to lead nowhere, especially when the said villain is powerful enough to best a Supreme Kai, but the line wasn’t expanded upon throughout the original Toriyama story. However, Old Kai finally confirmed in the Dragon Ball Super anime that it was Lord Beerus who had sealed him in the Z Sword so long ago. The old-timer’s concern had been raised after feeling Beerus awaken and begin a rampage of destruction, and he recounted the tale of his incarceration to Kibito Kai, talking up the fearsome power Beerus possessed and fretting over the future of the universe now that his former enemy had reemerged.
Akira Toriyama himself had previously made this revelation during a 2014 interview and added further context to Old Kai’s account, explaining the true reason behind Beerus’ actions. Toriyama noted that all the Supreme Kais and Gods of Destruction would hold strategy meetings every millennium and, during one such gathering, Beerus and Old Kai fell out over some “trifling situation.” Since destroying the Sacred World of the Kai would’ve been frowned upon, the hot-tempered Lord Beerus retaliated by trapping the then-Supreme Kai in the legendary sword.
As a sequel, Dragon Ball Super has changed and retconned more than a few Dragon Ball Z ideas, such as Saiyans being able to grow facial hair and the permanency of the Potara earring fusion. However, the Old Kai/Beerus connection differs in that it complements the original series, rather than changes it. Instead of rewriting what fans already thought they knew, this reveal simply filled in a pre-existing gap without any problematic contradictions. Even the line about Beerus not being as bad as Buu can be interpreted to mean villainy, rather than power, which makes complete sense.
Given the vague description of events, it’s possible that Beerus had ulterior motives for trapping the Old Kai. Whis once revealed to Goku that the lives of a universe’s God of Destruction and its Supreme Kai are linked – the death of one would kill both. Although the exact mechanics of this (admittedly unfair) system aren’t clear, it’s possible that preserving Old Kai in the Z Sword effectively meant Beerus could only die if some one defeated him, which was never likely to happen. Knowing this, Beerus could safely spend most of Dragon Ball sleeping peacefully.