Not many people know that the other name of Diwali is "Naraka Chaturdasi". In Dwapar Yuga, Lord Krishna, the ascendant of Lord Vishnu, killed the notorious demon Narakasura on the same day as that of Diwali.
Narakasura was the son of God Bhoomi and ruled the kingdom of Pradyoshapuram. Despite of a great parentage, he possessed devilish tendencies. Puranas have it that Naraka, son of Bhudevi, acquired immense power from a blessing given by Lord Brahma after a severe penance. Under his rule, the villagers suffered a lot of hardship as the demon tortured the people and kidnapped the women to be imprisoned in his palace with his invincible might. He used to loot the people of three worlds and due to the supernatural powers he had nobody could stop him. Not only he plundered their valuable things, but abducted the womenfolk too. There was a terror of Narakasura everywhere.
Once Narakasura attacked the heavens to get hold of the army of elephants of Lord Indra. he defeated Lord Indra and snatched away the magnificent earrings of Mother Goddess Aditi who was not only the ruler of Suraloka but also a relative of Lord Krishna's wife, Satyabhama. Narakasur also imprisoned sixteen thousand daughters of Gods and saints of marriageable age in his harem, and planned to marry them. This created chaos everywhere.
Indra being helpless before the demon, immediately went to Lord Krishna to ask for help. Krishna at that time was spending a leisurely time with his wife Satyabhama. Narakasura had a boon that he would face death only at the hands of his mother Bhudevi. In order to fulfill both his duties as a friend and as a husband he took Satyabhama at the battlefield. Satyabhama, was the reincarnation of Bhudevi, he made her his charioteer in the battle with Narakasura.
On his divine vehicle Garuda (a giant eagle), Lord Krishna set for the fort of Narakasura. His grand fort was guarded by a five headed demon Mura. With his flying discus, Krishna beheaded Mura and then challenged Narakasura to come to fight on the battlefield. When Krishna fell unconscious after being hit by an arrow of Narakasura, Satyabhama takes the bow and aims the arrow at Narakasura, killing him instantly. Later Lord Krishna reminds her of the boon she had sought as Bhoomi Devi. Narakasura was killed and his son Bhagdatta was taken by Krishna under his supervision on the request of Bhoomi Devi.
In order to save all those imprisoned ladies from embarrassment Lord Krishna accepted them all as his wives. As a symbol of the victory over Narakasur, Lord Krishna smeared his forehead with this demon king's blood. Then Lord Krishna returned home with his new wives early morning of the Naraka Chaturdasi day. The womenfolk massaged scented oil to his body and gave him a good bath to wash away the filth from his body. Since then, there is a custom to take bath before sunrise on the day of Naraka Chaturdasi, especially in the state of Maharashtra. It is said that the mother of the Narakasura, Bhoomi Devi, declared that the death of her son should not be a day of mourning but an occasion for celebrations. Since then people celebrate Choti Diwali with joy and fun every year.
The killing of Narakasura again takes us to the message that it is inevitable for virtue and truth to win over vice and evil. The same message Lord Rama portrayed by killing Ravana. Bhoomi Devi herself was happy of the killing of her Son. The message of Narakasura Chaturdashi Parva is that the good of the society should always prevail over one's own personal bonds. The Narakasura Vadh by Satyabhama could also be taken to interpret that parents should not hesitate to punish their children when they step in to the wrong path.. It is interesting to note that Bhudevi, mother of the slain demon Naraka, declared that his death should not be a day of mourning but an occasion to celebrate and rejoice. It is said Lord Krishna had an oil bath to rid himself off the blood spattered on his body when Narakasura was killed.
It is still a tradition among Hindus to clean the Krishna idols with hot water before burning the effigy of Narakasura. The tradition is followed and people offer prayers on the previous day of the Naraka Chaturdashi to the vessel in which water is being heated for having bath.