Asked to join in slaying King Soma, Mitra refuses as the friend of all, then yields; the cattle leave him, and the cup he shares with Varuna is mixed with milk.
An explanation of the Mitra-Varuna cup at the Soma pressing. The gods invite Mitra to slay Soma; he declines as friend of all, then, foreseeing that cattle will desert him, asks that his share be mixed with milk. Varuna helps on condition that one cup serve both. In a time without day or night the pair, granted that only the Indra-Vayu cup precede theirs, produce day (Mitra) and night (Varuna).
Soma was Vritra. After the gods slew him they told Mitra that he too was a slayer. He objected that he is everyone's friend; threatened with exclusion from the sacrifice he said he too slew. The cattle left him and the gods restored them by mixing milk into the Soma: the milk is Mitra's, the Soma Varuna's. The chapter opens with Mitra as priesthood and Varuna as ruling power, who could not succeed until he asked Mitra to go before him.
What is different here:
Deed already done; a threat instead of a bargain; milk given by the gods, not chosen; Soma equated with Vritra; adds the union of priesthood and ruling power; no day-and-night episode.
Sources: The Veda of the Black Yajus School (Taittiriya Sanhita), tr. A. B. Keith (1914); The Satapatha-Brahmana, tr. Julius Eggeling (Sacred Books of the East 12, 41)