Dragon Ball or the sacred dragon once been killed by Demon King Picolo. In the chapter of return of King Picolo. The Sacred dragon had been killed after he granted King Picolo wish. At this time no one could ever beat King Picolo, even Goku and Master Roshi.
King Picolo killed sacred dragon with hope that no one will ever use the wish to challenge him in the future. But the Sacred Dragon again has been given a life by kami after Goku ask if kami could bring back his friend who been killed by King Picolo. Instead of giving life back to his friends, kami give life to Sacred Dragon and the dragon will granted any wish including giving goku friends life back. It is quite weird but this is the story ....
After granted the wish to bring back his friends, Sacred Dragon turn to the Dragon Ball and flew separated araound the world.