Like most of us, Bhishma was also curious about the reason behind the rivalry between Karna and Arjuna. On the surface he would have also been confused considering Arjuna did nothing in his entire life to provoke Karna while Karna despite being many years older felt like competing against Arjuna and proving himself better than him was his sole life goal. He went to the extent that despite knowing that Arjuna was hist brother, he could not let the desire to kill Arjuna go.

So like any curious person, Bhishma wanted to find out and he asked Pulastya rishi for the reason behind this epic rivalry between the two great warriors. Pulastya rishi answered Bhishma's curiosity by narrating the story of their past births, which is recorded in Chapter 14 of Srishti Khanda of Padma Purana. This is that story.

After Mahadeva chopped of one of the heads of Brahmadeva, Brahmadeva in his anger took up the sweat on his forehead and struck it on the ground. From that perspiration emerged a being having rings, arrows, a great bow and one thousand armours.

Brahma ordered this being with thousand armours to kill Rudra. The being picked up the bow and started attacking Maheshwara. Maheshwara went to Bhagwan Vishnu asking him to kill the being emerged from Brahma's sweat.

Mahadeva presented the skull of Brahmadeva to Bhagwan Vishnu. Bhagwan Vishnu lifted his right arm forward. Mahadeva made a cut with his trident making blood flow into the skull. When the skull was filled with Bhagwan Vishnu's blood Mahadeva stirred it with his finger. When the blood was thus churned a being with strong shoulders having two quivers on his back emerged who was wearing finger protector and a crown.

*25. Seeing him Bhagwan Viṣṇu said these words to Rudra:

26-27. “O Bhava, who is this man (Nara) that has sprung up in the skull?” Hearing (these) words of Viṣṇu, Śiva said to him: “O Lord, listen; this is the man named Nara, best among those who know (the use of) great missiles. You called him Nara, so he will be (called) Nara.

28-30. Both Nara and Nārāyaṇa will be famous in the Yuga and in battles, in deeds helping gods and in the protection of people. This Nara, therefore, will be the friend of Nārāyaṇa. This one of great lustre will help you in killing the demons; he will be a sage in testing knowledge and a conquerer in the world. This is the fifth head of Brahmā, with a superior lustre.

31-33. Born from the combination of the three lustres that rose from the blazing lustre of Brahmā and the glance that I cast, he will conquer the enemy in battle. He will be terrible to those who cannot be killed by Indra and (other) gods and also to the others who will be invincible to Indra and (other) gods.”*

Mahadeva ordered Nara to fight and kill Sahasrakavacha, the one born from Brahma's sweat. They fought for two divine years. When Nara was about to kill Sahasrakavacha, Bhagwan Vishnu went to Brahmadeva.

*44-46. Madhusūdana (i.e. Viṣṇu) spoke, in alarm, these words to Brahma: “O Brahman, the one born of (your) perspiration was today hurled down by the one born of (my) blood.”

On hearing this Brahmā who (became) distressed said to Madhusūdana:

“O Hari, let this my man live (again) in this existence.”

Viṣṇu being pleased said: “It shall be so.”*

*47. Going to their battlefield and warding them off he said to both of them: “In the intervening period between the Kali and Dvāpara yugas, in your next birth there will be (a war); when the great war will take place I shall bring both of you together (to fight).”

48. Viṣṇu having called the lord of planets and the lord of gods told them: “By my command these two good men are to be protected by you.

49. O sun (having a thousand rays), this one born of the perspiration (of Brahmā) is to be descended on the earth as your portion at the end of the Dvāpara (yuga) for the success of gods.*

Bhagwan Vishnu then Instructed Indra as follows:

*59. When the Sun-god, the robber of clouds, diappeared (Viṣṇu) with his mind pleased said to Indra also:

60. “O Indra, this Nara sprung up from (my) blood due to my favour and who is a portion of me should be placed by you on the earth at the end of Dvāpara.

61. O magnanimous one, when the illustrious Pāṇḍu will obtain Pṛthā as his wife and so also Mādrī he will go to a forest.

62. A deer will give him a curse when he will be in the forest; with aversion produced by that he will go to Śataśṛṅga.

63. Desiring sons to be born on his wife (by another) he will say (so) to her. Then Kuntī unwilling (to do so) will say to her husband (i.e. Pāṇḍu):

64. ‘O King, I do not at all desire sons (begotten) by a mortal. O King, I desire sons from the deities by their favour.’

65. O Śakra then you should present Nara to Kuntī who would beseech you. O Lord of Śacī, do so by my direction.”*

In the end Bhagwan Vishnu concluded.

Viṣṇu spoke for taking down the burden of the earth: “O Lord, I shall take an incarnation in the mortal world for destroying Sun’s son and for your son’s success. I shall also act as (Arjuna’s) charioteer and bring about the destruction of the Kuru-family.”