After Raditz kidnapped Goku's son, Gohan, he forms a truce with Piccolo in order to defeat Raditz. After sacrificing his life during the battle, Goku trains with King Kai in the Other World. He teaches Goku the "Kaiō-ken", which multiplies his energy and strength but with possible strain to the body. It is also from King Kai that Goku learns his most powerful attack: the "Genki-Dama", an energy sphere created by gathering energy from surrounding animals, nature and humans. After being revived by the Dragon Balls, Goku faces off with the Saiyan prince Vegeta, who eventually becomes his greatest rival and another ally. On his journey to Planet Namek in order to aid his friends in gathering the Namekian Dragon Balls to revive those killed by the Saiyans, Goku fights the galactic tyrant Frieza, who destroyed the Saiyans' home planet and nearly the entire race. During his epic battle with Freeza, Goku becomes the first Saiyan in a thousand years to transform into a fabled Super Saiyan.