The Seven Star Warriors

The Seven Star Warriors can only occur when seven Saiyans who have at least reached the level of Super Saiyan absorb the Dragon Balls. This was told to Goku by the Elder Kai, stating that it happened before during a primitive war against the Namekians and Saiyans, before Guru (the Grand Elder) was born. Seven warriors became Super Saiyans and absorbed the Dragon Balls. Their power was legendary and unrivaled and it took the strongest Super Namekians to combat them. Goku and the others use this sometime in GT between Baby and Super #17. The advesary they use it against evil Super Namekians who invaded Earth. Trunks took the 1 star Dragon Ball, Goten took the two star, Vegeta took the three star, Goku took the four star, Gohan took the five star, Pan took the six star (she mastered Super Saiyan but never used it against #17 or the Shadow Dragons), and Future Trunks came back for the occasion and took the seven star. They attacked the Namekians, yet they weren't strong enough. They fused together and made the Super Shenron Warrior, who didn't even lift a finger, yet simply wished the Namekians away.