As the Taittiriya Samhita tells it
Taittiriya Samhita 6.4.8 · 3 min read
Soma was a king among the gods. Soma was also the plant whose juice the gods wanted more than anything else. To get the juice, the stalks had to be crushed. The gods did not hide from themselves what that meant. They called it by its plain name.
They went to Mitra.
"Let us slay Soma, the king," they said.
Mitra's name means friend, and he answered like one.
"Not I. I am the friend of all."
"We will slay him all the same," said the gods.
Mitra saw that the thing would be done whether he joined in or not. He saw something else as well. The text says he knew it beforehand: when I have done this cruel deed, the cattle will leave me.
"Then let me choose a boon," he said. "Let my share of the Soma be mixed with milk."
The gods granted it. So Mitra took his part in the pressing, and it happened as he had foreseen. The cattle turned from him and walked away.
"He was a friend," they said, "and he has done a cruel deed."
But there was milk in his cup now. Milk comes from cattle, and where the milk is, the herd is not far. By that mixing Mitra was joined to the cattle again. This is why, the priests say, the cup drawn for Mitra and Varuna is always mixed with milk. They add a warning for the man who pays for the rite. Anyone who offers Soma does a cruel thing of a kind, and cattle draw away from him too. The milk in that one cup brings them back.
The gods still needed help, and they went next to Varuna, who is a king.
"With you as our helper we will slay Soma, the king."
Varuna did not refuse. He named his price.
"Let me choose a boon. Let this cup be drawn for me and Mitra together."
So the cup belongs to the two of them and is never drawn for one alone. The priests take a rule of the world from it: it is with a king's help that a king is brought down.
One trouble was left. In that time, the text says, there was neither day nor night. Everything lay together, not yet told apart.
The gods came to the pair.
"Make this shine out for us," they said.
"Let us choose a boon," said Mitra and Varuna. "Let only one cup be drawn before ours."
That was granted too, and so at the Soma pressing the cup of Indra and Vayu is drawn first, and the cup of Mitra and Varuna comes straight after it.
Then they did what they had been asked. Mitra brought forth the day. Varuna brought forth the night. The world shone out, with light and dark each in its own place, and it has done so ever since. The priests say that when the shared cup is drawn, it is drawn for that shining.
So the friend of all keeps the day, and the cup he shares with the king of the night has milk in it, because he once tried to say no.
As the Shatapatha Brahmana tells it
Shatapatha Brahmana 4.1.4 · 2 min read
The Shatapatha Brahmana tells the same moment with a harder edge. In its telling the deed is already done.
Soma, it says, was Vritra. When the gods had slain him they turned to Mitra.
"You are a slayer too," they said.
Mitra did not like it. "Surely I am everyone's friend. If I stop being a friend I shall become the opposite of my own name."
The gods did not bargain. "Then we shall shut you out of the sacrifice."
That was more than he could bear. "I too slay," said Mitra.
At once the cattle went from him. "He was a friend," they said, "and he has become an enemy." He was left without a herd. So the gods mixed the Soma with milk and in that way gave him cattle again. Here the milk is a kindness from the others. Mitra did not think to ask for it.
The Brahmana then sorts the cup into two parts. People still say that he had no liking for the killing. So the milk in the mixture is Mitra's, and the Soma in it is Varuna's.
The same chapter tells how the two came to share a cup at all.
Mitra, it says, is the priesthood, the one who thinks a thing out. Varuna is the ruling power, the one who does it. In the beginning the two stood apart. Mitra could stand without Varuna. Varuna could not stand without Mitra. Whatever he tried on his own came to nothing.
So Varuna called to him. "Turn to me, so that we may be joined. I will set you in front. Whatever I do, I will do with you going before me."
"So be it," said Mitra.
From that day, the Brahmana says, Varuna succeeded in what he did, and one cup is drawn for the pair. It draws a rule from this for kings: a king who means to act should first find a priest to think with him.