As the Shatapatha Brahmana tells it
Shatapatha Brahmana 14.1.1, with Rigveda 1.116 and 1.117 · 3 min read
The gods once held a long sacrifice at Kurukshetra. Agni was there, and Indra, Soma and Vishnu, and all the rest. Only the two Ashvins were left out.
During that sacrifice a disaster happened, and the sacrifice lost its head. That is another story. What matters here is that the gods went on with their offerings afterwards and nothing came right, because they were working with a headless thing.
One person knew how the head could be put back. He was a sage named Dadhyanch, the son of Atharvan. He knew the pure heart of the sacrifice and how it is made whole again. The old texts call this knowledge the honey.
Indra knew that he knew. He came to the sage and spoke plainly.
"If you teach this to anyone else, I will cut off your head."
So Dadhyanch kept silent.
Word of it reached the Ashvins. "Dadhyanch knows how the head of the sacrifice is put on again," they said to each other. They went to him.
"We two wish to become your pupils."
"What do you want to learn?" he asked.
"The pure heart of the sacrifice. How its head is set back in place, and how it becomes complete."
"I cannot take you," said Dadhyanch. "Indra has told me that if I teach it to anyone, he will cut off my head. I am afraid he will do it."
"We will protect you from him," said the twins.
"How will you protect me?"
Now the Ashvins were physicians, and they had a physician's answer.
"When you have accepted us as pupils, we will cut off your head ourselves and put it away in a safe place. Then we will fetch the head of a horse and set it on your shoulders. With that head you will teach us. When you have finished, Indra will come and cut off the head that taught us. Then we will bring your own head back and put it on again."
Dadhyanch thought about it.
"So be it," he said.
He took them as his pupils. They cut off his head and laid it safely aside. They brought a horse's head and fixed it on him. And through the mouth of a horse the sage taught the two young gods the honey, all of it, holding nothing back.
When the teaching was over, Indra came. He kept his word and struck off the head that had spoken. It was the horse's head.
The Ashvins kept theirs. They fetched the sage's own head from where they had hidden it and set it on him again, and Dadhyanch stood up whole.
The poets of the Rigveda had sung of this long before. One of them, Kakshivat, said that he would proclaim this deed of the twins the way thunder announces rain. He sang of how Dadhyanch, by a horse's head, had told them of the honey. Another verse says that the horse's head itself spoke the words to them.
The Brahmana adds a rule of its own. This knowledge is not for everyone. A teacher should give it only to a pupil he knows, who has lived with him a full year.
As the Brihaddevata tells it: the head in the lake
Brihaddevata 3.18 to 24, with Rigveda 1.84 · 2 min read
The Brihaddevata is an old handbook that explains which god each hymn is for and adds the stories behind the hymns. It begins this one a step earlier and ends it a step later.
It was Indra himself, the handbook says, who first gave the secret to Dadhyanch. He was pleased with the son of Atharvan and taught him the spell that shows where the Soma lives. The sage shone brighter for having it.
Then Indra thought better of his gift.
"Do not speak of this honey anywhere," he told him. "If it is made known to anyone, I will not let you live."
The two Ashvins came to the sage quietly and asked for the honey in secret. Dadhyanch told them what the lord of heaven had said.
"Then let us receive it from a horse's head," said the Nasatyas. "Do it quickly. Indra will not kill you for that."
So he taught them through the horse's head. Indra took that head off with his thunderbolt, and the twins put the sage's own head back.
This is where the Brahmana stops, and the Brihaddevata does not.
The horse's head, struck off by the thunderbolt, fell a long way. It came down in the middle of a lake on a mountain called Sharyanavat. It did not rot there. It rose from the water and gave many kinds of gifts to living things, and then it sank again. The handbook says it lies under that water still, and will lie there until the end of the age.
The Rigveda has two verses that seem to belong to this ending. They say that Indra once went searching among the mountains for the horse's head, and found it at Sharyanavat. And they say that with the bones of Dadhyanch he struck down his enemies, ninety nine of them.
Indra had struck off that head himself. In the end he had to go looking for it.