Everyone knows the story of Shrikrishna's birth and the events in his parents' lives leading up to that. Kamsa out of fear for his life at the hands of the eighth son of Devaki decided kill all of them at childbirth.

Chapter 2 of Vishnu Parva of Shri Harivamsha Purana tells the backstory of the first 6 sons of Devaki and Vasudev, who were killed by Kamsa. This post is about them.

Kamsa ordered the ministers to be engaged in destroying the conception of Devaki. "Starting with the first, all the conceptions shall be destroyed as they are born. Where there is an apprehension about a danger, it should be destroyed at the root. Let Devaki move freely in the house, guarded by my men in disguise. When she becomes pregnant, she should be placed under guard. My women will track her pregnancy and we will do the needful at the time of delivery. Servants desiring my benefit shall guard Vasudeva in the women's quarters throughout day and night. Only such human efforts are possible by humans. But a powerful man like me can thwart the course of fate by certain means. Listen. With the chanting of mantra-s, appropriate use of medicines and using suitable tactics, fate can be made favourable."

Thus Kamsa began to make efforts to destroy Devaki's conception. He had understood everything from Narada. Frightened, he began to make plans for his protection.

Hearing about the efforts of Kamsa which will bring disaster, the invincible, valiant Bhagwan Vishnu contemplated as follows: "Kamsa will definitely kill the seven conceptions of Devaki. I should install myself as her eighth conception."

While thinking like this, Bhagwan Vishnu's mind was suddenly drawn to Patala, where danavas named Shadgarbha were undergoing austerities lying in water. These danavas who had powerful bodies, were bright like devas who drink amrita. They displayed vigour like devas in battle. They were sons of Kalanemi.