Super Dragon Ball Heroes is a breeding ground for new forms that can't be seen anywhere else. They have standard features like new Super Saiyan 3 and 4 transformations and fusions, but it gets crazier than that. They also have Super Android 17 after absorbing other androids, Baby merged with Hatchiyack, Golden Meta Cooler and Demon Gods. However, one of the crazier transformations is something that could only happen with the story SDBH is trying to tell.
Among the characters to get a new look is Majin Buu. This version of Buu, referred to as Xeno Majin Buu, receives some amazing powerups courtesy of the Dark Demon Realm. They make him more than strong enough to go up against fighters he wouldn't normally beat in the main Dragon Ball timeline.
The first of Xeno Majin Buu's powerups comes in the form of a Dark Dragon Ball. In SDBH, the Dark Demon Realm creates the Dark Dragon Balls with the intent of making a wish on them for their leader Mechikabura. Although the Dark Demon Realm is responsible for making these Balls, they still have to seek out and gather all seven of them like everyone else.
The Dark Dragon Balls are scattered across time and space and embedded in some of history's most notorious villains. Villains like Frieza and Cell who have Dark Dragon Balls embedded in them receive ungodly new power. With this new power, the heroes of their respective timelines who are supposed to win against these villains can no longer do so alone. They require the help of their Xeno counterparts; members of the Time Patrol who are also responsible for stopping the Dark Demon Realm's plans. The effort needed to stop opponents with Dark Dragon Balls in them speaks to the amount of power the Balls grant their hosts.
Xeno Majin Buu is naturally one of the villains to host a Dark Dragon Ball. This version of Kid Buu has the Three-Star Dark Dragon Ball embedded in his chest. Not only is he too strong for his own timeline to handle, but he's able to fight off Xeno Goku when he comes to stop him.
Unfortunately, Xeno Majin Buu is no match for his next opponent, Demon God Xeno Dabura. While the Demon King is utterly humiliated by Fat Buu in the original timeline, Xeno Dabura's Demon God form gives him the edge even against this powered-up version of Kid Buu. In fact, the difference in power is so great that he is able to defeat Xeno Majin Buu effortlessly. Not only does the Demon God redeem himself for his loss in the "Buu Saga," but he also proves to be a powerful new threat that even Xeno Goku can't handle.
This is where Xeno Majin Buu makes his comeback. While Xeno Dabura is distracted by Xeno Goku, the Majin gets the jump on him and absorbs him. He becomes Dark Demon God Xeno Buu, the strongest version of the Majin yet. Even Xeno Vegito is only able to fight an even match with this Buu. Just as Dabura redeems himself by beating Buu, so too does Buu redeem himself by matching Vegito.
As if Dark Demon God Xeno Buu isn't a long enough moniker, the Majin acquires even further power. Besides Xeno Dabura, he also finds and absorbs Xeno Janemba, who himself has a new Evil Demon Form thanks to his Five-Star Dark Dragon Ball. With all of these powerups stacked up on top of one another, Xeno Majin Buu becomes more than a match for anybody in the Time Patrol.
It's only when Xeno Majin Buu goes up against the Demon Gods that he really seems to have trouble. This makes sense from a storytelling standpoint, as the Demon Gods are the primary antagonists of the Dark Empire Saga. Xeno Majin Buu, in the context of the narrative, is just a minor antagonist who happens to have a way of getting much stronger than he rightfully should be. Regardless, all the issues that this relatively minor antagonist causes are a testament to how powerful the Majin Buu has become.