What this song renders
The documented spine is short and solid. Olympias was the daughter of Neoptolemus I of the Molossians, of the Epirote royal house known as the Aeacids, which claimed descent from Achilles; she married Philip II of Macedon in 357 BC. Plutarch opens the Life of Alexander with exactly that pairing — Achilles on her side, Heracles on his. Her birth name is given in the tradition as Myrtale; ‘Olympias’ is one of several names she carried across her life.
The cult is documented too. Plutarch (Alexander 2.7–2.9) describes the ecstatic Dionysian rites of the Macedonian and Epirote women and says Olympias, more than any of them, carried great tame serpents in the dances, coiling out of the ivy-wands and around the sacred spears. What the track renders is that practice taken at face value: a genuine religious office, not the enchantress the later hostile writers built out of it.
What is invented is the interiority. No source records what the girl in Epirus thought of the marriage, and no source gives her a word before Macedon. The song’s claim — that she came to Philip already carrying the two things that would outlast him — is the album’s reading of a documented lineage and a documented cult, spoken in a first person the record never granted her.
The lineage, the cult and the marriage are documented; the serpents are Plutarch’s, not the album’s. Everything interior is invention — the voice, the defiance, the reading of the match as a thing done to her. The foreshadowed son is dramatic irony, not prophecy: nothing in the sources has her predicting Alexander.