text: SB 6.9.18: That very fearful demon, who was actually the son of Tvashta, covered all the planetary systems by dint of austerity. Therefore, he was named Vritra, or one who covers everything.
  Vritra became the head of the asuras (portrayed as inherently malicious here, as opposed to the Vedic version, in which they may be benevolent or malevolent). He renounced his dharma – duty – to do good unto others and turned to violence, battling with the devas. Eventually, he gained the upper hand, and the devas were frightened of his evil might. Led by Indra, they approached Vishnu for help. He told them that Vritra could not be destroyed by ordinary means, revealing that only a weapon made from the bones of a sage could slay him. When the deities revealed their doubts about the likelihood of any ascetic donating his body, Vishnu directed them to approach the rishi Dadhichi. When approached by the deities, Dadhichi gladly gave up his bones for the cause of the good, stating that it would be better for his bones to help them attain victory than to rot in the ground. The devas collected the bones and Indra crafted the Vajrayudha from them. When they engaged Vritra again, the battle lasted for 360 days before Vritra breathed his last.