Killing of Narakasura
Unable to bear the tyranny of the demon, the celestial beings pleaded with Lord Krishna to save them from his torture. But Naraka had a boon that he would face death only at the hands of his mother Bhudevi. So, Krishna asks his wife Sathyabhama, the reincarnation of Bhudevi, to be his charioteer in the battle with Naraka.
When Krishna fell unconscious after being hit by an arrow of Naraka, Sathyabhama takes the bow and aims the arrow at Naraka, killing him instantly. Later Lord Krishna reminds her of the boon she had sought as Bhudevi. The Narakasura Vadh by Sathyabhama could also be taken to interpret that parents should not hesitate to punish their children when they step in to the wrong path.
The message of Naraka Chaturdashi Parva is that the good of the society should always prevail over one's own personal bonds. 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 Naraka was killed.
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. Hindus light fireworks, which are regarded as the effigies of Narakasura who was killed on this day.