The text at the centre of this service is an oddity among the world's liturgical books: not a hymnal or a prayer collection but a legal code. The Videvdad, the law against the daevas, is cast as a dialogue in which Zarathushtra asks and Ahura Mazda answers, and its chapters legislate the great Zoroastrian concerns of purity: the treatment of the dead, the contagion of corpse pollution, the offences against water, earth and fire, the penalties and the means of purification, including the elaborate nine-night cleansing. Around this code stand chapters of myth, the story of Yima and his underground refuge against the world-winter, the catalogue of the sixteen lands created good and counter-created with plagues, material that has made the book a primary source for Iranian legend as well as law. That such a text is recited entire as a liturgy says something essential about Zoroastrian ritual theology: the sacred word is operative, and reciting the law against pollution is itself an act against pollution, not a reading about one.
Philologists note that the Vendidad's Avestan is later and rougher than the Gathic hymns, the work of a priesthood composing in a partly remembered sacred language, and that it is the only major Avestan book to survive complete, precisely because its ritual use kept every chapter in living transmission. The scriptural text itself is treated in the atlas among the Avestan books; the service described here is its performance, the point where the law book becomes an act.