Lord Brahma imparted the knowledge of the gods to his eldest son Atharvan. Atharvan’s son Dadhyanch, on growing up, renounced the world and became a sage. He was very knowledgeable but he could not match the knowledge that his father possessed. In order to acquire that extra knowledge, he decided to penance. So he continued to pray till Lord Indra appeared in front of him.
Indra asked, “Why have you been doing penance for so long? What is it that you want?”
Dadhyanch replied, “I want to learn all the sciences that you know.”
Indra agreed to teach him health science initially but only on the condition that he would not share the knowledge that was imparted with anyone else. “If you do not listen to me, I shall chop off your head,” Indra warned.
When Dadhyanch agreed, Indra taught the celestial health science to him. One day, the Ashwin twins heard about the boon Dadhyanch had received and began to pester him to teach it to them.
Dadhyanch clarified that he could not do so as Indra had made him promise not to reveal it to anyone. In case he were to do so, Indra would behead him.
The Ashwin brothers persisted, “Do not worry on that score. We shall handle Indra. You tell us the secret and we shall keep it within us.”
Dadhyanch, not very strong of character, relented to the demand and revealed all that he had learnt about health science. After acquiring the necessary knowledge, the Ashwin twins chopped off Dadhyanch’s head and replaced it with that of a horse, so that Indra would not get angry.
When Indra heard what Dadhyanch had done, he visited Dadhyanch that very night and chopped off his head. At this, the Ashwin twins promptly restored Dadhyanch’s real one. All were happy at this turn of events.
So long as Dadhyanch lived on earth, he could control the asuras (demons) but once he left for heaven, the asuras began to ride roughshod over the land. Indra went to fight the asuras but failed to dislodge them. He said to the other gods, “If only Dadhyanch had left something of his own alive, I could have created a weapon out of it to defeat the asuras.”
At this, the Ashwin twins told Indra about the horse’s head which they had dumped in the lake and which had been with Dadhyanch for some time.
Indra recovered the horse’s head from the lake and out of its bones carved out weapons that could counter ninety strategems that the asuras tried against Indra. But Indra foiled all of them. This way peace was restored to the land.