He was Krishna's maternal uncle. He was a descent of a demon named Kaalanemi. His father Ugrasena was king of all sects of the Yaadavas. But, later, Kamsa dethroned and jailed his father to ascend the throne.
when Vasudeva married Kamsa's step-sister Devaki, Kamsa himself drove their procession carriage. at that time, a formless voice from the sky declared that Devaki's eighth pregnancy results in Kamsa's death. Hearing that, Kamsa tried to kill Devaki,but Vasudeva dissuaded him. As a measure of safety for him, Kamsa imprisoned Devaki and Vasudeva. He killed all Devaki's six sons while they were all young children.
But, at the time of Devaki's seventh pregnancy, the foetus was transpositioned into the womb of Vasudeva's another wife Rohini. That made people believe that Devaki suffered a miscarriage.
As the eigth son of Devaki, Vishnu himself incarnated. But, the boy was smuggled secretly in the midnight to a cowherd Nanda's house and the latter's newly born daughter, the divine mystic Maaya in fact, was kept in place of Devaki's son. When Kamsa new that eith baby of Devaki was born, he was all eagerness and rushed to kill the baby. But, the baby flew off from his hands and warned about the existence of Kamsa's killer elsewhere.
Since then, Kamsa send many demons to kill all the children aged below two years. He also sent many demons to kill Krishna but they all were slain by Krishna. Doubting that his killers caould be Balaraama and Krishna with Nanda, Kamsa invited the two young boys to his court on the pretext of some sacrifice Dhanuryaaga.
Kamsa employed his cruel tusker Kuvalayaapeedam to kill Krishna at the entrance of the regal hall. But Krishna killed the tusker. Kamsa also urged his wreslers Chaanoora and Mushtika to kill Krishna and Balaraama but those two too were slain. Finally, Krishna killed Kamsa and restored the throne to Ugrasena. He released his parents from prison.
Krishna incurred a big trouble by killing Kamsa. Kamsa was son-in-law of Jaraasandha, the mighty emperror of Magadha. Jaraasandha's two daughters Asthi and Praasthi were Kamsa's wives. Enraged with the killing of his son-in-law, Jaraasandha invaded Krishna's city Madhura incessantly. After eighteen invasions, Krishna shifted his residence from Madhura to Dvaaraka inthe middle of the sea.
"After praying to Naaraayana, Nara, Sarasvathi and Vyaasa, then one should read the Jayam."
DEDICATED TO MY DEAR NIECE VYSHNAVI KRISHNA