Karna Trades His Armour and Earrings for Indra's Shakti
Indra, disguised as a Brahmin, comes to Karna and asks for his divine armour and earrings — the only things that make him invulnerable. Karna knows exactly what he is giving away. He asks for the invincible shakti in return, cuts the armour from his own body with a sword, and hands it over smiling — earning the name Vaikartana and reducing himself to a mortal who can now be killed.
Content in his mind, Karna asked for the invincible shakti. "O Vasava! In exchange for my armour and earrings, give me the invincible shakti that kills large numbers of enemies on the field of battle."
For an instant, Vasava thought about this in his mind. With the spear in mind, he then spoke these words to Karna.
"Give me the earrings and the armour from your body. O Karna! Take the shakti from me, but on one condition. When I am fighting with the daityas, this invincible shakti is released from my hand and having killed hundreds of enemies, returns again to my hand. O son of a suta! But in your hands, it will kill one powerful enemy who roars and blazes. Then, it will return to my hand."
Karna replied: "I wish to kill only one enemy in a great battle. He roars and blazes and is the source of fear for me."
Indra said: "You kill one roaring and powerful enemy in battle. But the one you seek to kill is protected by a great-souled one who is known as the unvanquished boar, Hari and the inconceivable Narayana by those who are learned in the Vedas."
Karna replied: "O illustrious one! Nevertheless, give me the invincible shakti, capable of slaying one brave person, so that I can kill the powerful one. I will slice off the earrings and armour and give them to you. But after this, let my wounded limbs not look loathsome."
Indra said: "O Karna! You will never look loathsome. You do not utter a falsehood and your body will not be scarred. O supreme among eloquent ones! O Karna! You will again possess the complexion and energy of your father. Your complexion will again become like his. But if you unleash this invincible weapon in a fit of fury, when you possess other weapons, there is no doubt that it will descend on you."
Karna replied: "O Shakra! As you have told me, I will release Vasava's weapon only when I confront supreme danger. I promise you this truthfully."
He then accepted the flaming shakti.
He grasped his sharp sword and began to cut up his entire body. On seeing Karna cut up his own body, the gods, humans, danavas and masses of siddhas began to roar, because despite the pain, there were no distortions on his face. Celestial drums were sounded and divine flowers were showered down from above, at the sight of Karna, the brave man, smiling repeatedly as he cut up his own body with the sword. Having sliced off the divine armour from his body, while it was still wet, he gave it to Vasava. Then he sliced off and gave the earrings. Because of this deed, Karna came to be known as Vaikartana.
Shakra smiled at his deception, thinking that he had accomplished the objective of the Pandavas, and subsequently, soared up to heaven. However, he ensured Karna obtained fame in the world.
Having heard that Karna had been deceived, all the sons of Dhritarashtra were miserable and their insolence was shattered. But on learning about the state that the son of the suta had been reduced to, the Parthas were happy in the forest. Aranyaka Parva, Chapter 591