Sri Krishna Avatara is one of the Avatara of Lord Vishnu. The story of Krishna Avatara is also mentioned in Padma Purana.
Once, goddess Parvati expressed her desire to know about all the incarnations of Lord Vishnu.
Continuing with the various incarnations of Lord Vishnu, Lord Mahadeva told Parvati —
Vasudev was the descendant of Yadu. His father was Devameedha. He was married to Devaki–the daughter of Ugrasena. Ugrasena also had a mighty son named Kansa. Kansa was extremely affectionate towards his sister Devaki. After her marriage, while Devaki was going to her husband’s house on a chariot being driven by Kansa, suddenly a heavenly voice was heard.
‘Kansa, you would meet death at the hands of Devaki’s eighth son.’
Kansa became very scarred and wanted to kill his sister then and there. But, Vasudev requested Kansa to spare Devaki’s life and promised to hand over any child that she gave birth to. Thus Kansa imprisoned both Vasudev and Devaki. In course of time, Devaki gave birth to seven sons and Kansa killed each of them.
Lord Sri Hari manifested himself when Devaki conceived for the eighth time. It was the month of Shravana and the day was ‘Krishna-ashtami’ when Sri Hari took his incarnation at midnight. Soon after taking his incarnation he gave a glimpse of his divine form to Vasudev and Devaki. On the instruction of Sri Hari, Vasudeva carried the infant to Nand’s house. He had no problem in moving out of the prison as all the guards had fallen asleep due to the illusion of Sri Hari.
On seeing the swift currents of overflooded yamuna, Vasudev became worried. But the level of river Yamuna receded as soon as Sri Krishna touched the river with his feet. Sheshnag protected Sri Krishna from heavy rain with his thousand hoods. When Vasudev reached Nand’s house he found that Nand’s wife– Yashoda had given birth to a baby girl. Yashoda was sleeping at that time and Vasudev had no problem in exchanging Sri Krishna with that baby girl. Vasudev then returned to the prison and laid down the baby girl beside Devaki.
The baby-girl started crying and the guards immediately woke up. They informed Kansa about the birth of Devaki’s eighth child. Kansa tried to kill that baby girl by smashing her head, but she escaped from his hands and flew up in the sky. She thundered—‘Your death is certain as your assasin has already taken birth.’ Kansa was terrified at the prospect of his death. He instructed his soldiers to kill all the infants’ whereever they were to be found in his kingdom. He also ordered the release of Vasudev and Devaki, as there was no point keeping them in captivity. Meanwhile, Sri Krishna grew up under the guardianship of Nand and Yashoda.
One day Kansa sent a wicked ogress named Putna to kill Sri Krishna. Putma had applied poison on her breasts and tried to kill Krishna by breast-feeding him. Sri Krishna sucked her breast with such ferocity that she died. Thus, Sri Krishna gave the first glimpse of his divinity to the inhabitants of Vajra. Sri Krishna accomplished many such divine deeds during his childhood. He grew up to be a mischieveous child and used to play numerous pranks on the fellow Gopis. He used to steal their butter and curd.
One day, all the Gopis complained to Yashoda about Sri Krishna’s misdeeds. Yashoda became angry and tied him with a wooden-mortar in order to prevent him from troubling the Gopis. She then placed that mortar between two trees so that Krishna could not move. But, Sri Krishna toddled away uprooting both the huge trees.
Once, a demon named Bakasur disguised himself as a huge heron and tried to kill Sri Krishna. But, Sri Krishna hit him with a stone with such force that he met an instant death. This way Sri Krishna had killed many ferocious demons while he was still a small child. Some of them well Kaliya–the serpent etc, Dhenukasur, Arisht, Keshi etc.
A significant incident had occurred during Krishna’s childhood. People of Vraja were devotees of Indra and used to worship him. Sri Krishna stopped this tradition and encouraged people to worship Govardhan mountain. Indra became extremely furious and caused incessant rain for one week. Sri Krishna protected the people by lifting Govardhan mountain and shielding them from the continuous downpour. At last, Indra accepted defeat and eulogized Sri Krishna.