Kamsa was born to King Ugrasena a Yadava king and Queen Padmavati. However, out of ambition and upon the advice of his personal confidantes, Banasura and Narakasura, Kamsa decided to overthrow his father and install himself as the King of Mathura. Therefore, upon the guidance of another advisor, Chanur, Kamsa decided to marry Asti and Prapti, the daughters of Jarasandha, King of Magadha.
After a heavenly voice prophesied that Devaki’s eighth son will slay him, he imprisoned Devaki and her husband Vasudeva and killed all their children; however the eighth son, Krishna, an avatar of the god Vishnu, was transported to Gokul, where he was raised in the care of Nanda, the head of cowherds. Kamsa sent a host of demons to kill the child Krishna, all of whom Krishna killed. Finally, Krishna arrived in Mathura and slew his uncle Kamsa.
In reality, Kamsa was not the biological son of Ugrasena. The Brhad Bhagavatamrta references the Sṛṣṭi-khaṇḍa (48–51) of the Padma Purana describing that after Padmavati’s marriage with Ugrasena she stayed a short time in the house of her father, King Satyaketu. At that time a demonic messenger of Kuvera’s named Drumila (or Gobhila) became attracted to her. He saw Padmavati, as she was bathing. Overcome with lust, he came to her disguised as Ugrasena, and asked for sexual union with her. Thinking that her husband had returned from war, she yielded. That night they became intimate. During coitus, excited Drumila lost his self-control and resumed his true, demonic form. However, Padmavati was so attracted to him that she didn’t protest about it. As a result of this illicit love making, Padmavati conceived. She feared that Ugrasena may disown her as she was impregnated by a demon. But Ugrasena accepted her and she gave birth to Drumil’s son Kamsa. Later Padmavati gave birth to four sons of Ugrasena. In fact, Kamsa in his previous birth was a demon called Kalanemi, who was slain by Lord Vishnu.
In childhood, Kamsa was trained by the other Yadavas, who were famous warriors. Kamsa acquired Jarasandha’s attention when the latter tried to invade Mathura. Kamsa single-handedly routed Jarasandha’s army. The latter was impressed and made Kamsa his son-in-law. With Jarasandha’s support, Kamsa became even more powerful. He vanquished the very gods Indra, Varuna and Kubera in battles and forced Indra to rain on his kingdom.
During his wedding in Mathura, Jarasandha brought over his army to escort the Princesses Asti and Prapti. Using the army of Magadha as his political cover, Kamsa overthrew his father after he refused to voluntarily retire from his position. This was done within the confines of the royal palace and the public was not informed. After Ugrasena failed to show up for public events, Kamsa announced his coronation.
Kamsa was told, in a prophecy, that the eighth child of Devaki will kill him. Hearing that, he wanted to kill Devaki, but Vasudeva managed to save her life by promising Kamsa that he (Vasudeva) himself will deliver Devaki’s all their children to Kamsa. Kamsa accepted that promise and spared Devaki because she herself was not a threat to him. In the confines of the prison, Devaki repeatedly conceived and cruel Kamsa murdered the first six children.
Just before the birth of seventh child, Lord Vishnu summoned Goddess Yogmaya, an eight-handed woman holding different weapons in her hands and wearing different colored garments. Shri Hari or bhagawan Vishnu asked her to transfer the embryo of Shesha Naaga from Devaki to Vasudeva’s another wife Rohini in Gokulam. This child was named Balarama, Shri Krishna’s elder brother. Whereas, Lord shri Hari-Vishnu Himself, was soon to appear as the eighth son of Devaki, he ordered Yogamaya (who, shall be known with different names by her devotees such as Durga, Bhadrakali, Narayani, Chandika, Vaishnavi, Sharada, Ishaani, Vijaya, Chin Bhavani, Amba and Ambika) to take birth from the womb of Yashoda. According to Shri Hari-Vishnu’s orders, Yogmaya transferred Shesha from the womb of Devaki to the womb of Rohini. Facilitating God Vishnu’s descent or avatar, Yogmaya (as the controller of the darkness and ignorance) had put the guards of Kamsa to sleep or a state of trance. At this time, Vasudev, obeying Shri Hari’s order, took BalKrishna to Nand- Yashoda’s house, bringing back the Baby girl, Durga, who was the incarnation of Yogmaya. Presuming this baby as Devaki’s eighth child, Kamsa was about to kill her by crashing her down on the ground, but the girl slipped out of his hands. Taking her cosmic form, the eight-handed Durga warned Kamsa, “The eighth child, who shall kill you, has been born. He is in Gokul!”The seventh child, Balarama, was saved when he was moved to Rohini’s womb. The eighth child born to Devaki and Vasudeva was Krishna. Krishna was saved from Kamsa’s wrath and raised by Vasudeva’s relative Nanda and Yasoda, a cowherd couple.After Krishna grew up and returned to the kingdom, Kamsa was eventually killed by Krishna, as was originally predicted by the divine prophecy, and Ugrasena reinstated as King of Mathura.