The dragon at the Theban spring is the god's own property — "a dragon, which some said was the offspring of Ares, guarded the spring" — and killing it is a debt even a founder must pay [The Library (Bibliotheca), BOOK III, ch. IV]. "Cadmus killed the dragon, and by the advice of Athena sowed its teeth"; but sowing does not settle the blood: "But Cadmus, to atone for the slaughter, served Ares for an eternal year", and "the year was then equivalent to eight years" of ordinary reckoning [ibid.]. The tabu's long arm shows a chapter later, at the story's far end: Cadmus is "turned into a serpent and sent away by Zeus to the Elysian Fields" [The Library (Bibliotheca), BOOK III, ch. V]. Frazer's note there preserves Hyginus's reading of that ending as sentence, not mercy: the serpent shape was "a punishment inflicted by Ares on Cadmus for killing his sacred dragon", the one "which guarded the spring at Thebes" [ibid., editor's note]. Kill the god's dragon and the god collects twice — eight years of servitude now, and at the end of life you are remade in the shape of the thing you killed.