The Yadavas had the custom of having many wives. Vasudeva was a much married man (he already had seven other wives and had a good time - Yadavas, unlike the Pauravas believed in polygamy) and while he waited for a one to one with Kamsa on the matter of his cruelty, chanced to see Kamsa's cousin Devaki, barely eighteen then, (and from what we hear - pretty hot and sexy) and got a good hard-on seeing her. Knowing however, that he was not a Tom Cruise, when Kamsa finally came (after raping another virgin, he told him , "You know Kamsa, we have been such great friends. Why don't we become relatives as well."
Kamsa:"That's great. Which of your sisters can I marry? I have heard a lot about Kunti, the one your father gave away. Heard she has turned into a great beauty. It will be great to have her." Vasudeva: "No, no, Kamsa, I cannot get Kunti married. She is now the adopted daughter of Kuntibhoja, but..."
Kamsa: "..What about your other sisters then? Can't I marry any of them? They are all hot, from what I hear."
Vasudeva: "But they all are already married. And moreover, I came to talk about my marriage, not yours."
Kamsa: "Your marriage? But you are already married, and that too seven times, aren't you, while I am married only twice - and that too largely being political wedocks - to the daughters of Jaransadha? What are you proposing though, by the way?"
Vasudeva: "You know about your cousin, the daughter of your father's younger brother, Devaki. Has not she just turned eighteeen? And what better husband for her than your very good friend, me. I will keep her so happy (and she will keep me so happy as well), and it will be a grand wedding. Moreover, the kingdoms of Shoorsen and Ugrasen will unite."
Kamsa (pondering for a while): "What you say makes sense, Vasudeva. We can arrange for your marriage to Devaki. Is it why you came all the way? To ask for Devaki's hand?"
Vasudeva: "And there is another small matter, Kamsa. The citizens of your kingdom are tired. You are all powerful, as you are, but could you be a little less cruel to your subjects?"
Kamsa: "Are you telling me to change my intrinsic nature? Then that is not possible. And the marriage can be re-negotiated if you want."
Vasudeva: "No no, forget it. I had come to ask for Devaki's hand."
Kamsa: "It is yours, my friend - and infact is all of her body (evil grin). Let us have a grand wedding."
Vasudeva: "But what about Devaki's consent. Will she agree?"
Kamsa: "How does her consent matter? She will marry wherever I tell her to marry, and that's that. " (Calls his minister): "Begin the preparation for the grand wedding."
And so there was a great wedding. Kamsa proved to be the ideal brother, hosting the lavish affair. It is said it could put Laxmi Niwas Mittal's daughter marriage to shame. Great many maidens were given as gifts to the guests, and all citizens of Mathura were to provide all their produce to cater for the wedding. All was going well for Vasudeva - the wedding was complete, and Kamsa was drawing the chariot to take the two when tragedy stuck, through an Aakashwani, which said: "Kamsa, the eighth son of Devaki will be a cause of your downfall." Hearing this, Kamsa pulled out his sword and proceeded to kill Devaki herself, but was stopped by Vasudeva, who promised to send the eighth born of Devaki to Kamsa as soon as he was born, for Kamsa to kill him. However, Kamsa, being the cruel ruler that he was, also demanded that all of Devaki's sons be brought to him, which was agreed to immediately by the horny Vasudeva.
Anything I can do to let me screw your cousin sister." And so off they went, Devaki and Vasudeva, to the kingdom of Shoorsen, where Devaki lived as the eighth wife of Vasudeva. And though Vasudeva divided his time among his wives, he was a bit more affectionate towards Devaki, who got pregnant in quick time. A son was born, who was carried off to Kamsa, who mercilessly killed the child. As time went by, Kamsa got more and more threatened by the yet to be born child, and killed off six of Devaki's sons, as soon as they were born. By this time, Kamsa had also managed to annex Shoorsen's and other Yadavas kingdoms for himself. He even took full control of the kingdom of Mathura, taking his own father as a prisoner and also made Vasudeva and Devaki prisoners. He had them put under the darkest cell of his prison, and had his best security guards to guard them from escaping. What really rattles everyone in this story is, as to why did he allow Devaki and Vasudeva to be together all this while - to let them produce six offsprings? Well, after much research, I have come to the conclusion that the Aakashwani mentioned just Devaki's son and she might have had other kids, who were not technically Vasudeva's. You get what I mean, right? So well, Kamsa was not as stupid as might look at first sight. He allowed them their pleasures, but kept his eye firmly on Devaki's stomach. (That's a metaphor, btw). And so there were news that Devaki was pregnant with her seventh child, but with a bit of divine intervention involved, the womb was transferred from Devaki to Rohini, the eldest of Vasudeva's wives, who had escaped Kamsa's rage by living as a common villager in Nand Gaon, a suburb of Mathura. (Heard of surrogate mothers- we Indians had it all before the west even started). Meanwhile it was reported to Kamsa that Devaki's seventh child was born dead. As it turned out, the kid that grew up in Rohini's womb grew up to be Balarama, considered to be an incarnation of the SheshNag. Meanwhile, Devaki got impregnated again (with the eighth child), around the same time that Kunti was carrying Arjun in her womb, and this time the kid was born in the prison itself, but somehow (with divine help again) managed to escape from prison as soon as he was born. He was taken by Vasudeva, (who came back to the prison to decieve Kamsa) , to the house of Nand in Nand Gaon, where he was named Krishna, and adopted by Nand's wife, Yashodha. Krishna is believed by many to be an incarnation of the God Vishnu himself. As it passed, Kamsa knew he had been fooled, and went on a manhunt to capture Krishna, and sent many demons as well, but Krishna, helped by Balarama, managed to kill them all, and finally killed Kamsa himself - to fulfil the prophecy. And Ugrasen again became the new king of Mathura, while Krishna and Balarama became princes. Order was restored - for now. Krishna and Balarama grew up as Yadava princes, the cousins of Pandavas through Kunti.