Devraj Indr is the giver of Rains which he provides through his weapon the Vajra, the thunderbolt. The Vajra in his hand is the strongest weapon and has a prominent role in all the victories of Devraj Indr, along with the bestowing of rainfall to Earth for sustenance of mankind.
The Vajra has a unique story behind its making. It was not out of a boon or a celestial power that Devraj Indr had acquired the weapon, but with the superior sacrifice of Rishi Dadhichi. The story of Rishi Dadhichi is mentioned in the Bhagavat Puran. Rishi Dadhichi was the son of Atharvan and Chiti. Atharvan is said to be the orator of the Atharvan Veda by which he got his name too.
During the time of the wars between the Devas and the Asuras, a powerful demon named Vrita defeated Indr through sorcery and deceit. As Indr was the God of rains, Vrita took away all his divine source of delivering rains and made the Earth a barren land. All living beings were deprived of water and started perishing. This worried the devas and all sought the help of Bhagwan Vishnu.
Bhagwan Vishnu listened to all their pleas, but said that it was impossible to defeat Vrita with the weapons they had with them. He asked them to approach Rishi Dadhichi and take his help in creating weapons which would be invincible and lead to the defeat of Vrita. All the devas reached Rishi Dadhichi’s ashram and sought his help. Rishi Dadhichi listened to the problem of the devas and understood the role he had to play.
He called for Devraj Indr and said to use his bones which are hard as a diamond and unbreakable to make weapons for the Devas. For the weapon needed by Devraj Indr, he asked them to make a weapon out of his spinal cord bone which would be the greatest weapon ever created. The devas were shocked listening to this and said that they cannot take the life of the great Rishi for their cause. But Rishi Dadhichi convinced them that sacrificing life for the greater good is the noblest cause of all. He says that no life can survive without water and it was very important that the devas regain back their power for preserving Dharma.
Creating a sacrificial fire, Rishi Dadhichi enters the fire and immolates his body in the fire. The devas offer salutations to his great sacrifice. Then they collect the ashes of his bones and Vishvakarma designs powerful weapons out of them. He creates the Vajra from the spinal cord of the Rishi and it becomes the greatest power yielded by Indr. Thus, by gaining powerful weapons from Rishi Dadhichi’s ultimate sacrifice, the Devas wage war with the Vritasur and his demon army. In the forceful battle which happened, Indr strikes Vrita with his Vajra and slays him. The he releases the water kept captive by striking the Vajra in the clouds and the Earth becomes replenished once again with water. Indra gains back his throne and peace and Dharma are restored back in the three worlds.
By offering his body as a weapon to Devraj Indr, Rishi Dadhich helped the Devas not only through his powers, but also through the ultimate sacrifice of his life for Dharma and made his name immortal among our great and revered sages.