The answer to this question starts with the story of Indra.
Indra & The Slaying of Vritra
Every great civilization has a good dragon-slaying story. In Hinduism, one of the most ancient and primal archetypal pieces of lore is the story of Indra and the slaying of Vritra.
The story appears in varied forms across the Vedas, Mahabharata, and the Puranas.
At its core, the tale begins with Indra killing Vishvarupa, the son of Tvashta. Tvashta is of mysterious origins, a Prajapati who was also an asura. In some accounts, Tvashta is the son of Vishwakarma, the divine architect among the devas. In some, he is Indras’s own brother.
Vishvarupa and Indra were enemies from the beginning. Vishvarupa had three faces. With one face, he used to recite the Vedas, with the second he drank alcohol, and, with the third, he observed the world. He was engaged always in rigorous tapas.
The depth of his tapas disturbed Indra, who sought to keep a firm grip on his throne. One day, Indra killed Vishvarupa.
When Indra slew Vishvarupa, he was afflicted with the demerit of brahmanahatya (killing a brahmana). Indra lost all of his energy. Specifically, the Markandeya Purana says that Indra’s energy departed from him and entered Dharma.
Eyes red with rage at the loss of his son, Tvashta tore off a lock of his hair, offered it as an oblation into the fire, and out of that offering was born Vritra. He emerged from the southern side of the sacrificial altar. Vritra’s body grew by the length of a bow day by day, like arrows released in the four directions. He resembled a burnt hill with hair and a beard the color of melted copper, eyes as piercing as the afternoon sun. He danced and shouted wildly, making the entire surface of the earth tremble as if from an earthquake. With each yawn, he appeared to swallow the entire sky. He licked up all the stars in the sky with his tongue and ate the skies with his long, sharp teeth. He became known as Vritra, the one who covers everything.
In the Puranic accounts, Indra and the devas went to Vishnu for guidance on how to defeat Vritra. Vishnu tells them that Vritra can only be killed by a weapon made from the bones of a sage. The great…