Karkotaka Bites Nala and Transforms His Form
Nala, having rescued Karkotaka, wishes to release the serpent — but Karkotaka tells him to walk and count his steps. At the tenth step, the serpent bites him, and Nala's form instantly transforms into something hideous while the serpent takes his shape.
Nala held the serpent in his hands, ready to let him go. But Karkotaka spoke again.
"O Nishadha! Walk on and count your steps as you proceed. O great king! I will ensure your great welfare when you do this."
Nala obeyed. He began to walk, counting each step. At the tenth step, Karkotaka bit him.
Instantly, Nala's form changed. He looked down at his own body and saw it was deformed — twisted, unrecognizable. He stopped, astounded. When he looked up, he saw that the serpent had assumed his own form.
Karkotaka spoke to console him. "I have brought this change in your form so that no one may recognize you. I have done this so that the person who has injured you with this great distress will unhappily reside inside you, stung by my venom. As long as he does not free you, he will suffer from my venom all over his body. He will be in misery as long as he lives there.
"You were innocent and did not deserve to be harmed. I have protected you from the anger and envy of the one who has cheated you. You will no longer face any fear from enemies who have fangs or from those who have knowledge of the brahman. You will suffer no pain on account of the poison. You will always be victorious in battles."
The serpent's venom had entered the body of Kali — the embodiment of the Kali Yuga — who had possessed Nala and driven him to ruin. Now it was Kali who would suffer, trapped inside Nala's transformed body, stung by the serpent's poison.
Nala was no longer recognizable as a king. He was now a deformed man, free to move unseen through the world. Aranyaka Parva, Chapter 360