kamsa had a favorite sister, who was devaki, kRSNas mother. kamsa was getting her married to vasudeva and he was happily taking the newly married couple in a procession. When a divine voice mocked him, saying "Foolish kamsa the 8th child of devaki is going to be the end of you." kamsa was furious and took his sword our to kill his own sister. Her newly married husband begged that she be spared, but kamsa imprison both of them and they will give all their children into the custody of kamsa.
kamsa grudgingly agreed because the towns people had been witness to this whole drama. He did not want a PR nightmare on his hands.
So they were put in jail. The 7 children that will be born to devaki, before kRSNa, had requested God that they didn't want to be born on earth. If they were born they wanted to be killed as soon as they were born, so that they could return to their original abode.
A divine sage comes to the house of kamsa, and tells kamsa that the Gods are really clever beings and the divine voice didn't tell him, how they were counting down to the 8th child. So kamsa hatches a plot to kill all devaki's babies. So everytime a baby is born to devaki, the guards would tell kamsa and he would come and take the baby and smash it against the walls and kill it.
Before the 8th child was born viSNu/God appeared to devaki and vasudeva and foretold his coming. He also said that there was a small-time chieftain named nanda, who had a wife named yashoda. God asked vasudeva to take him and put him beside yashoda in the dead of the night and get yashoda's child and put it beside devaki.
When kRSNa was born an ignorant sleep overtook the land and all the guards fell asleep. The prison doors opened on their own and vasudeva took kRSNa and crossed the flooded yamuna river and went to nanda's tent. Nanda was the chief of a group of nomadic tribesmen who tended to cows and bartered milk and diary products in exchange for food. They were just a little above the poverty line.
The current gypsies of Europe who are loathed and racially discriminated to oblivion are probably descendents of tribes such as kRSNas.
So vasudeva goes with the newborn in a basket on his head, all the way to nanda's place and crosses the flooded yamuna river. The yamuna river raises above the head of vasudeva and after touching the feet of God resides completely.
Once vasudeva is back with the other child. The guards wake up and alert kamsa. kamsa comes and devaki begs him to spare the newborn as it is a girl baby and is of no harm to kamsa. He refuses to listen, snatches the baby from devaki and throws the baby onto the stone floor. The baby flies up instead of down and shows it's true form the Goddess durga and says that my husband is growing in this land and will kill you soon and disappears.
A very disturbed kamsa comes back to the palace and orders the slaughter of all newborns in the land less than one year old. He sends ghoulish assassin demons who slip silently in the night killing children.
One of the assassins was a demonic being named putana. Who smeared poison on her nipples and fed the young babies saying that her breast milk had divine powers and killed them.
She came to kRSNa's home and tried to feed kRSNa. kRSNa suckled the life force out of her and killed her. This was when was 3 days old or something.
Next kamsa sent a demon named triNavarta who could change his appearance at will. He came in the form of a cart on the 11th day and was planning to run over kRSNa. kRSNa's mom yashoda had temporarily put kRSNa under this cart as there was a naming ceremony of kRSNa going on their house. kRSNa was kicking his hands and legs (supposedly) un-coordinatedly like little babies do. His left little leg made contact with the cart like trinavarta and the cart shattered into pieces and trinavarta fell dead. Of course everyone comes running now. A bunch of kids who were tasked with looking after the baby tried to tell the adults that they saw the uncoordinated kicking of the baby kill the demon, but all the adults thought the kids were nuts.
To be continued ... (Hopefully)