"It is said in other puranas that Narakasura was the son of Dharitri, the earth, by the Lord Himself. But he became a demon due to the bad association of Bana, another demon. An atheist is called a demon, and it is a fact that even a person born of good parents can turn into a demon by bad association. Birth is not always the criterion of goodness; unless and until one is trained in the culture of good association, one cannot become good."
Srimad-Bhagavatam 3:3:6 Purport
"An instance of astonishment in devotional service by indirect perception occurred when Maharaja Pariksit heard from Sukadeva Gosvami about Krsna's killing Narakasura, who had been fighting Krsna with eleven aksauhini divisions of soldiers. Each division of aksauhini soldiers contained several thousand elephants, several thousand horses and chariots and several hundreds of thousands of infantry soldiers. Narakasura possessed eleven such divisions, and all of them were throwing arrows toward Krsna, but Krsna killed them all, simply by throwing three arrows from His side."
Nectar of Devotion, Chapter 46
"Bhaumasura was also known as Narakasura, for he happened to be the son of the earth personified. When he saw that all his soldiers, commanders and fighters were killed on the battlefield by the strokes of the weapons of the Personality of Godhead, he became exceedingly angry at the Lord. He then came out of the city with a great number of elephants who had all been born and brought up on the seashore. All of them were highly intoxicated. When they came out, they saw that Lord Krsna and His wife were beautifully situated high in outer space just like a blackish cloud about the sun, glittering with the light of electricity. The demon Bhaumasura immediately released a weapon called Sataghni, by which he could kill hundreds of warriors with one stroke, and simultaneously all his assistants also threw their respective weapons at the Supreme Personality of Godhead. Lord Krsna began to counteract all these weapons by releasing His feathered arrows. The result of this fight was that all the soldiers and commanders of Bhaumasura fell to the ground, their arms, legs and heads separated from their trunks, and all their horses and elephants also fell with them. In this way, all the weapons released by Bhaumasura were cut to pieces by the reaction of the Lord's arrows.
The Lord was fighting on the back of Garuda, and Garuda was also helping the Lord by striking the horses and the elephants with his wings and scratching their heads with his nails and sharp beak. The elephants were feeling much pain by Garuda's attack on them, all were all dispersing from the battlefield. Bhaumasura alone remained on the battlefield, and he engaged himself in fighting with Krsna. He saw that Krsna's carrier, Garuda, was causing great disturbance to his soldiers and elephants, and in great anger he struck Garuda with all his strength, which defied the strength of the thunderbolt. Fortunately, Garuda was not an ordinary bird, and he felt the strokes given by Bhaumasura just as a great elephant feels the impact of a garland of flowers.
Bhaumasura thus came to see that none of his tricks would act upon Krsna, and he became aware that all his attempts to kill Krsna would be frustrated. Yet he attempted for the last time, taking a trident in his hand to strike Him. Krsna was so dexterous that before Bhaumasura could touch his trident, his head was cut off by the sharp Sudarsana cakra. His head, illuminated by earrings and helmets, fell down on the battlefield. On the occasion of Bhaumasura's being killed by Lord Krsna, all the demon's relatives began to scream in disappointment, and the saintly persons began to glorify the chivalrous activities of the Lord. Taking this opportunity, the denizens of the heavenly planets began to shower flowers on the Lord."
Krsna Book, Chapter 59
"....Krsna, at the request of Indra, kills the demon Narakasura, the son of the earth personified, along with the demon's associates, headed by Mura. The earth personified offers prayers to Krsna and returns to Him all the paraphernalia that Narakasura has stolen. Krsna then bestows fearlessness upon the son of Narakasura and marries the sixteen thousand princesses whom the demon kidnapped."
Srimad-Bhagavatam 10th Canto Summary
"When sixteen thousand girls were kidnapped by the demon Bhaumasura, they prayed to Krsna, and therefore Krsna went to the demon's palace, killed the demon, and delivered all the girls. But according to the strict Vedic system, if an unmarried girl leaves her home even for one night, no one will marry her. Therefore when Krsna told the girls, "Now you can safely return to your fathers' homes," they replied, "Sir, if we return to the homes of our fathers, what will be our fate? No one will marry us, because this man kidnapped us."
"Then what do you want?" Krsna asked. The girls replied, "We want You to become our husband." And Krsna is so kind that He immediately said yes and accepted them.
Now, when Krsna brought the girls back home to His capital city, it is not that each of the sixteen thousand wives had to wait sixteen thousand nights to meet Krsna. Rather, Krsna expanded Himself into sixteen thousand forms, constructed sixteen thousand palaces, and lived in each palace with each wife."
Teachings of Queen Kunti, Chapter 12
Bhaktivedanta Book Trust. Excerpted from text and purport of HDG A.C. Bhaktivedanta Swami Srila Prabhupada.