Lord Krishna, the protagonist of the Mahabharata, was the eighth avatar of Vishnu and is worshipped by Hindus worldwide. Krishna's elder brother, Balarama, was an avatar of Shesha, the serpent on which Vishnu rests. Krishna and Balarama were the biological sons of Vasudeva and Devaki. Devaki had a wicked cousin named Kamsa.
After usurping the throne from his father, Ugrasena, Kamsa ruled over Mathura. On the evening of Devaki’s wedding to Vasudeva, she, Vasudeva, and Kamsa were returning home in a chariot. Suddenly, a voice from the sky proclaimed, "O foolish Kamsa, the eighth son of Devaki and Vasudeva will kill you."
Kamsa wanted to murder Devaki right there, but her husband, Vasudeva, promised to surrender all their children to Kamsa as soon as they were born. Kamsa imprisoned Vasudeva and Devaki and killed their first six children shortly after birth.
Devaki prayed for the survival of their seventh child. Before the child was born, the goddess Yogmaya transferred it from Devaki’s womb to the womb of Rohini, Vasudeva's first wife. The seventh child of Vasudeva and Devaki was thus born to Rohini and was named Balarama.
The eighth child born to Devaki and Vasudeva in prison was Krishna, who survived and later killed Kamsa.