The Mahabharata - pg. 122

Krishna was deeply moved and he consoled the weeping Daupadi. He said: "Those who tormented you will be stricken to death in the bloody quagmire of a lost battle. Wipe your eyes. I solemnly promise that your grievous wrongs shall be amply avenged. I shall help the Pandavas in every way. You will become an empress. The heavens may fall, the Himalayas may split in twain, the earth may crumble or the boundless sea may dry up - but, I tell you verily, my words shall stand. I swear this," and Krishna took a solemn vow before Draupadi.
This vow, it will be seen, was in perfect accord with the purpose of the Lord's avatars, as declared in scriptures: "For protecting the righteous, for destroying the wicked and for firmly upholding the law, I am born on earth age after age."