Once there was a tribe of Dark Bakugan existed in Ancient Times. These Bakugan exist in Gundalia because when the two cores of Vestroia were split, they got blasted into another dimension. Over time numbers vanished until Darkus Linehalt was left, he soon became Ren's partner. Ren's family was in charge of the Dark Bakugan. The strange thing is that no one, even the Dark Bakugan themselves know what these Forbidden Powers are until Linehalt accidentally releases it in his rage, despite that the Dark Bakugan and Ren's family were forced to live underground for centuries. It is unknown if the Dark Bakugan are only Darkus attributed.
Ren had been given a chance from the Emperor to lead the advanced Gundalian troops on Earth. Ren is only doing this mission to ensure freedom from the underground darkness where he and Linehalt have emerged from. This mission he has involves recruiting strong Brawlers to fight Neathia with him and seizing The Element. However, it is revealed that his true mission was to prevent Linehalt's power from being unleashed.
The Forbidden Power
The power is released in episode 25 and is seemingly strong enough to destroy worlds, however Colossus Dragonoid (the Transformation Hub for Dragonoid Colossus) stopped Linehalt before the power could reach that level.