Dragon Ball Z Kai Season 5 Episode 33

Birth! Combine Super Warrior, His Name is Gotenks!!

After some dubious persuasion from Goku, Old Kai reveals he has the ability to draw out someone's true power beyond their limits, though this turns out to be a lengthy process that can take over a day to complete. Meanwhile, Goten and Trunks try out the Fusion Pose for real, but an error in the Fusion Pose results in the merged fighter, Gotenks, ending up with a failed transformation. After another failure, the pair eventually perform a successful fusion and transform into the true Gotenks who, confident that he can defeat Buu with his current power level, goes off in search of Buu, despite Piccolo's warnings.