Article one's account of Angra Mainyu rests on the Gathas, seventeen hymns that describe the two primal spirits' choice in only a few compressed verses. Zoroastrian scholastic literature composed many centuries later, chiefly under the Sasanian empire and after, filled that sparse description in with an elaborate narrative the Gathas themselves do not contain, and the gap between the two stages is worth marking clearly.
The Bundahishn, a Pahlavi compilation of cosmology whose surviving form dates from the early Islamic period though it draws on older material, narrates Ahriman's assault on Ohrmazd's creation in extended detail: an initial period of cosmic combat, Ahriman's corruption of each element of the good creation in turn, sky, water, earth, plants, the primal bull and the first human, and a bounded span of mixture between good and evil running toward the eventual renovation, frashokereti. None of this narrative sequence, with its specific stages and timescale, is present in the Gathic material, which states the moral choice between the two spirits but does not narrate a developed cosmic history of their conflict.
The contrast is a useful check on a habit worth flagging generally: treating a late, systematised scholastic account as though it were the tradition's original teaching. The same caution about the tidy nine-world scheme reconstructed from a much later Norse source applies here in parallel form: an ancient tradition's earliest surviving voice states a doctrine sparsely, and a much later literate class, writing under different conditions, expands it into the fuller account most later readers actually encounter.