Indra Tests Arjuna and Promises Weapons
After the lokapalas depart, Indra smiles and tells Arjuna he had known him before. When Arjuna asks Indra to be his preceptor for learning weapons, Indra tests him — suggesting he might use celestial weapons against humans. Arjuna vows he will only use them to ward off other weapons, and Indra, satisfied, reveals the test and commands him to go to his abode to learn all divine weapons.
Maghavan, lord of the gods, ascended his splendid chariot. The illustrious one, the destroyer of the enemies of the gods, smiled and spoke these words:
"O Dhananjaya! I had known you even before you arrived here. Later, I will reveal myself to you. O Pandava! You have earlier bathed in the tirthas. You have earlier tormented yourself with great austerities. You will therefore go to heaven. You will have to practise supremely terrible austerities again."
Then the illustrious one recounted all the austerities that would have to be performed. "On my instructions, Matali will take you to the world of the thirty gods. You are already well known to the gods and the great-souled sages."
At this, Arjuna told Shakra: "O illustrious one! Please show your favours to me. O lord of the thirty gods! I seek you as my preceptor, so that I may learn about weapons from you."
Indra replied: "O son! O scorcher of enemies! You will perform terrible deeds. The reason for which you wish to acquire weapons will be satisfied."
Then Arjuna said: "O slayer of enemies! I will never use those celestial weapons against humans, except to ward off their weapons. O lord of the gods! Therefore, give me those divine weapons. I will later obtain the worlds that weapons can attain."
Indra replied: "O Dhananjaya! I uttered those words in order to test you. Your words are worthy of someone who is my son. O descendant of the Bharata lineage! Go to my abode and learn all the weapons there — those of Vayu, Agni, Varuna, the masses of Maruts, of the Sadhyas, the grandfather, the gandharvas, the serpents and the rakshasas, those of Vishnu, the nairritas, and my own."
Having spoken those words to Arjuna, Shakra disappeared. Aranyaka Parva, Chapter 461