On this day, the victory of Lord Krishna over the asura King Narakasura is celebrated.
Narakasura, was the powerful warrior son Bhudevi had prayed Bhagwan Vishnu for. He ruled the kingdom of Pragjyothishapuram (in modern day Assam, also once know as Kamarup), regularly troubled the Devatas and disturbed the penance of the sages. Narakaasura had also kidnapped and terrorized the gopis of Vrindavan.
Narakasura would raid and plunder the Kingdoms of the three worlds. He did not even leave the Women and would kidnap them for his own personal harem. Narakasura heard that Indra, the king of the devas, had thousands of divine elephants in his army. Now Narakasura, greedy that he was, wanted to possess everything, so he attacked the heavens.
Indra was helpless as he watched Naraka's minions plunder the heavens. Naraka himself began pursuit of the devas. While pursuing the devas, a glimmering object far away caught his eye. Upon inspection, he realised that the glimmering object was mother Aditi's (the mother of the devas) earrings. He assaulted mother Aditi and grabbed her earrings.
Tired of this harassment, Indra and the other Devatas approached Lord Krishna and pleaded with Him to protect them from the asura King.
Indra reached Krishna's palace when Krishna was with Satyabhama. He told Krishna about the happenings and begged him for his help.
But the demon king could only be killed by a woman. So Lord Krishna asked His wife, Satyabhama, the reincarnation of Bhudevi, to be His charioteer in the battle with Narakasura.
Krishna summoned his mount Garuda (a giant eagle) and made way towards Naraka's fortress. An impenetrable barrier of magic mountains surrounded Naraka's fortress. The mountains were such that a barrier would come up from any side that Krishna tried to enter the fortress. Krishna, unperturbed, hurled his mace at the barrier and shattered the entire mountain range in one blow. A downpour of mystical weapons rained down on them. Krishna fired multiple arrows and destroyed all the weapons. In this manner Krishna destroyed countless other mystical barriers and finally reached Naraka's fortress.
Naraka's palace was guarded by the five-headed demon Mura. Mura hurled countless weapons at Krishna, but Krishna shot each one down with his bow and arrow. Then Krishna used his sudarshan Chakra and hurled it towards Mura dislocating Mura's five heads. Mura fell to the ground, dead.
Krishna challenged Naraka to battle and in the fierce encounter that ensued Krishna fainted. Satyabhama took Krishna's place and killed Naraka in a fierce battle. Only a woman could kill this demon and satyabhama did it. Lying on his deathbed on the battlefield, Narakasura begged for Lord Krishna's and Satyabhama's mercy. Hearing his entreaties, Bhudevi declared that his death should not be a day of mourning but an occasion to celebrate and rejoice. Bhudevi then sang hymns in praise of Krishna and begged him to take Naraka's son Bhagdatta under his protection. Krishna placed Bhagdatta on the throne and then freed all of Naraka's prisoners. The devas showered Krishna with flowers from the heavens. Bhagadatta was a great devotee of Lord Vishnu and he obtained a great weapon called Narayanastra from him. He was also a very skillful in the art of training Elephants for war. Bhagadatta later fought on the Kauravas side with his huge Elephant battalion and Kirata army (Chinese mercenaries) in the Mahabharata War and died at the hands of Arjuna.