The 14th phase of the dark moon, is celebrated as Naraka Chaturdasi in commemoration of the slaying of Narakasura who was causing menace to the people, by Krishna and His wife Satyabhama.
In South India, Deepavali is celebrated as Naraka Chathurdasi. Naraka was an Asura who lived about 5100 years ago. Narakasura ruled from his kingdom of Pradyoshapuram. His rule was a misery to the people of his land.
Krishna and his wife Satyabhama slayed Narakasura and freed people from his tyranny. This event of vanquishing Narakasura is celebrated as Naraka Chathurdasi. Chaturdasi is the 14th phase of the moon and is the night before Karthika Amavasya, the day of Deepavali.
It is for this reason that Deepavali is celebrated as a festival of victory of good over evil.