Arjuna Accepts the Preceptor's Fee from Indra
Harivahana — Indra himself — tells Arjuna that the time has come to pay his preceptor's fee. But before revealing the task, he makes Arjuna promise to do it. Arjuna agrees, and learns what the king of the gods requires: the destruction of thirty million nivatakavachas, the invincible danavas who live along the ocean shore.
Arjuna had spent years in Indra's heaven, learning weapons from the gods themselves. He had mastered the divine astras, earned the trust of the king of the gods, and become something beyond human — immeasurable, unassailable, incomparable in battle.
Harivahana — Indra — touched his head with his hands and spoke.
"Today, the masses of gods are incapable of vanquishing you in battle, not to speak of humans of the world who have not perfected their souls. You are immeasurable and unassailable. You are also incomparable in battle."
Then, with his body hair standing up, the god said again: "O brave one! There will be no one who will be your match in fighting with weapons. You are never distracted. You are skilled. You are truthful and in control of your senses. You are like a brahmana. You are skilled in the usage of weapons. O extender of the Kuru lineage! You have obtained ten weapons and five. O Partha! Know that you have no equal in all the five ways. O Dhananjaya! You know how to employ, restrain, return, pacify and counteract them. You know everything about atonement and repulsion. O scorcher of enemies! The time has now come for you to pay your preceptor's fees. Promise to pay it and then I will tell you what you have to do."
Arjuna did not hesitate. "If it is in my power to do it, then consider it done."
Bala and Vritra's slayer smiled. "There is nothing in the three worlds that is impossible for you. The danavas named nivatakavachas are my enemies. They live in an inaccessible spot, along the shores of the ocean. They are thirty million in number and they are identical in form, strength and radiance. O Kounteya! Destroy them there. That will be the preceptor's fee."
Then Indra equipped him. He gave him the immensely radiant and divine chariot, driven by Matali, yoked to horses whose hair was as beautiful as the feathers of peacocks. He tied an excellent diadem on Arjuna's head. He gave him ornaments for the body, similar to those he himself wore, and supreme, beautiful, impenetrable armor, pleasant to the touch. Arjuna strung Gandiva with a bowstring that does not decay.
He was ready. Aranyaka Parva, Chapter 462