The twelfth adhyaya of Agni Purana has described the summary of Harivamsha. In his description Agni had said that among the Yadavas, Vasudeva was the foremost and Krishna was the eighth child of Vasudeva and Devaki. In this chapter Lord Agni has described the birth of Krishna in the womb of Devaki. He had said that Krishna was exchanged with the daughter of Yoshadha for the fear of Kansa. In the twelfth chapter it has been described by Agni that a prophecy was related to Kansa which told him about his future destroyer. On hearing this Kansa had send the she-demon Putana to kill all the children around.

In the chapter the childish freaks of Lord Krishna and Balarama in Gokul has also been described by the God of Fire. There have been ample instances which talks about the early prowess of Lord Krishna, the naughtiness of the young Lord has also been well described.

It has been said that Krishna at an early age had destroyed Putana, the serpent Kaliya, the demons Dhenuka, Gardhava and many more. Agni had described that it was Krishna who had revived the Vedic sacrifices. The Lord had been to Mathura with Valarama and had killed Kansa in a single combat.

Other parts of the twelfth chapter have described how Lord Krishna had defeated Jarasandha and had annexed his kingdom with that of Vasudeva.

Lord Agni had said that it was Krishna who had found the city of Dwarka and had married sixteen thousand wives. The God of Fire had also said that Lord Krishna had forcibly carried away from heaven the mountain of gems, the Parijata Tree and had planted them in the garden of Satyabhama and had received lessons from the holy sage Sandipani, and had brought back to life the son of sage Sandipani.

Lord Agni has also given a vivid description of the happy life of Krishna and Rukmini and had also described the innumerable progeny that he had had. It was also said that Krishna was the destroyer of the pride of the Kauravas of Mahabharata.

Finally the twelfth adhyaya ends with the words of Agni which says that as one hears the story of Hari or Krishna one is able to see peace in life and is also able to gain the desired objects.