Some TMNT stuff was always meant for grownups.
The man known only as Bane was born in a prison on the Caribbean island of Santa Prisca, where he honed himself to become a master of many forms of combat and developed an unparalleled intelligence. Using a super steroid known as "Venom", Bane fought Batman and broke his back.
Bane is one of the Arkham inmates mutated by Shredder and Ra's al Ghul. Bane becomes a mutant elephant and quickly apprehended Batman and Damian Wayne, but Batman broke free by throwing electrified batarangs in Bane's face.
During the final battle, he was among the combatants whom Splinter fought, and was defeated. It was later confirmed that he would revert back to normal in a week's time.
A short time after the defeat of Shredder and Ra's al Ghul, Bane took the latter's place as the new head of the League of Shadows. He was just emerging from a Lazarus Pit under Gotham when Donatello opened a portal between dimensions, accidentally swapping himself with Bane.
Bane immediately began taking over the fragmented Foot Clan, and within a week chaos had spread over New York. However, being in an alien dimension meant that he did not have access to Venom, so he enlisted Baxter Stockman to find a way to synthesize more, in order to use it on Bebop, Rocksteady and the Foot Clan.
He also laid a trap for Batman - who had followed him to this dimension - and the Turtles, ordering them to serve him or die.