Background of Yudhishthir-Nahusha samvad:
Once Bheem was roaming in the forest. Suddenly a large serpent attacked him and Bheem was seized in its grip. He tried his best to make himself free but all his might went in vain. The serpent told him, "I'm hungry, I shall eat you!" Actually that serpent was King Nahusha, the ancestor of Kurus, who fell from Heaven for his extreme pride and misbehaviour with Rishi Agastya. With Agastya's curse, Nahusha became a large serpent and currently living in this jungle.
Bheem understood, he would not be able to escape. He would have to die. Then he said, "Who can avoid destiny! Otherwise why would all powers of arms be lost like this? But I don't worry for myself, my only sorrow is, my brothers still are deprived of their kingdom, and living in this forest! And when they will get the news of my death, they will lose all effort to regain the kingdom, because it's me who always keep reminding them about their duty of winning kingdom back! I know Arjun will not mourn my death because he is self-controlled, but mother will break down, and so as Nakul-Sahadev! That's my only grief at this moment!"
Yudhishthir was in their hut then. Suddenly he experienced several bad omens around him, and could not find Bheem in hut. He asked Draupadi and got to know that Bheem had gone out long time ago. Yudhishthir immediately ordered Arjun to protect Draupadi and the twins to protect the Brahmins. He left the hut to search for Bheem.
Yudhishthir noticed Bheem's foot-prints, and the trees uprooted by him while hunting, thus he followed the right track and discovered Bheem, motionless, coiled in a serpent's large tail. He understood what happened.
Yudhishthir told the serpent, "Please let my brother go, I shall arrange another food for you." Nahusha replied, "No, this is my food for today, as he himself has come near my mouth. And you flee from here as soon as you can, otherwise I will reserve you for my tomorrow's meal!"
Yudhishthir did not get frightened. He asked the serpent, "Who are you, large serpent? Pandu's son Yudhishthir is asking you, who are you? Are you any God or a demon? It's not so easy task to seize my brother Bheem! And do tell me what food should I give you to make my brother free."
Nahusha seemed impressed with Yudhishthir's courage and firmness. He replied, "I'm Nahusha, your ancestor." Then he narrated his own story of Agastya's curse. And finally said, "If you can answer to my few questions then I will let your brother go!"
Yudhishthir humbly replied, "Ask me whatever you like! I will try to give answer if I can."