What this song renders
The episode the vow answers is in Herodotus (1.211–213). Cyrus, advised that the Massagetae had never tasted wine, left a camp stocked with it and lightly guarded. The detachment Tomyris had sent under Spargapises overran it, fell to feasting and drink, and was slaughtered or captured in their stupor — her son among the prisoners. When Spargapises sobered and grasped what had happened, he asked to be freed of his bonds, and the moment he was loosed he killed himself.
Tomyris’s answer is the most quoted thing about her. Herodotus (1.212) gives her a message to Cyrus before the great battle: that he is ‘insatiable of blood,’ that she will give him his fill of it though he conquer her. The wineskin and the severed head come after the fighting (1.214) — but the oath is made here, in the cold space between her son’s death and the army she will throw at the Persians. The sun is named as witness because, in Herodotus 1.212, she swears ‘by the sun, the lord of the Massagetae.’
What the song renders is not the battle and not yet the wineskin — those are later. It is the declaration: a mother who has stopped reacting and started planning, holding the religious frame Herodotus gives her (the sun as the one god who sees) and turning it into an instrument of vengeance. The eating-and-drinking imagery — Cyrus binge-eating kingdoms, then made to drink his fill — is the song’s own framing of a charge Herodotus puts in her mouth: that he could never get enough of blood.
The vow is documented — Herodotus 1.212 gives Tomyris the ‘insatiable of blood’ warning and the oath sworn by the sun, the Massagetae’s god, before the battle. The wineskin and the head come after (1.214). What is staged is the moment of the oath itself; the words are dramatised but the gesture and the deity are Herodotus’s. See Truth & Legend.