The Kalakeyas, a tribe of demons, and Vritra fought with Indra and defeated him.
Indra, followed by all the gods who were under his protection, went to Brahma for help and advice.
Brahma told them to seek the sage Dadachi on the banks of the River Saraswati.
The sage was performing a severe penance but willingly helped Indra. From out of his body he removed all the bones and handed them to Indra.
With those bones a mighty six-edged weapon was made called the Vajra.
When released, the weapon would make a thunderous noise and destroy the most powerful enemy. Indra used this weapon to destroy Vritra.
He retained the weapon after his victory.