The greatest hero in Dragon Ball GT is actually the super-powered amalgam of two legacy Dragon Ball characters, Goku and Vegeta, who fuse to become the immensely strong Super Saiyan 4 Gogeta–though in one Dragon Ball story, that ultimate hero is twisted into an absolutely terrifying multiversal threat.
In the final episodes of the anime Dragon Ball GT, the Z Fighters are facing a dark, mystical villain known as Omega Shenron. Omega Shenron is an entity created from all of the dark energy that accumulated from selfish wishes made upon the Dragon Balls throughout the course of the Dragon Balls’ existence. Using the dark magic of the Dragon Balls, Omega Shenron entered the mortal world with the goal of conquering it–defeating anyone who thought themselves strong enough to challenge him. While Super Saiyan 4 Goku and Vegeta weren’t powerful enough to defeat Omega Shenron separately, SSJ4 Gogeta was plenty strong enough to get the job done, becoming Dragon Ball GT’s greatest hero in the process.
In Super Dragon Ball Heroes: Dark Demon Realm Mission! Chapter 12 by Yoshitaka Nagayama, the Time Patrol–an interdimensional police force consisting of alternate versions of classic Z Fighters who are charged with maintaining the proper flow of time in every universe–travels to the Dragon Ball GT universe during Super Saiyan 4 Gogeta’s climactic battle with Omega Shenron. Just as Gogeta was landing the final blows, the evil forces of the Dark Empire Army arrive, confirming the Time Patrol’s suspicions of a dark, timeline-disrupting presence in this world. The Dark Empire is an organization of demonic gods who wish to invade the mortal realm and take over. In that effort, the Dark Empire Army traveled to Dragon Ball GT’s world and used their mystical powers to corrupt the minds of every living thing there, including SSJ4 Gogeta–turning him into Dark Gogeta.
When Dragon Ball GT’s Gogeta becomes corrupted by the Dark Empire Army, he is transformed into a horrifying beast with unimaginable power. Dark Gogeta takes on the look of an almost Venom-like monster with razor-sharp teeth exposed through a sinister grin, a long grotesque tongue, and pitch-black goo encompassing his entire body. While Dark Gogeta was an immediate threat to the Time Patrol as well as his homeworld, the Dark Empire Army planned on using him in their conquest to conquer the entire multiverse, and he quickly proved he had the power to be a great asset in that dark mission. Not only did Xeno Goku of the Time Patrol have to fuse with Xeno Vegeta and become their own version of SSJ4 Gogeta, but Xeno Gohan had to unlock Super Saiyan 4 as well to match the raw power emanating from this demonic beast—powers that clearly revealed the "hero's" dark side and transformed him into a threatening force.
Luckily, Dragon Ball GT’s Gogeta was able to break free from the Dark Empire’s control with the help of the Time Patrol. However, if the Dark Empire held their grip over Gogeta’s mind–a being that is strong enough to take on another SSJ4 Gogeta as well as SSJ4 Gohan simultaneously–then Dragon Ball GT’s greatest hero would have remained a terrifyingly powerful and nearly unbeatable threat to the Dragon Ball multiverse.