The fullest telling of Coyolxauhqui's story is in the Florentine Codex, the sixteenth-century compilation made under the Franciscan Bernardino de Sahagun from Nahua testimony. On the mountain Coatepec, "Serpent Hill," the goddess Coatlicue is sweeping when a ball of feathers drifts down and she tucks it into her waist; by it she conceives. Her daughter Coyolxauhqui, discovering the pregnancy and taking it for a disgrace, rouses her four hundred brothers, the Centzon Huitznahua, and leads them up the mountain in arms to kill their mother.
As they climb, the child in the womb speaks to reassure Coatlicue, kept informed by one of the brothers who has turned traitor to the plot. At the moment of the attack Huitzilopochtli is born, fully grown and fully armed, carrying the fire-serpent weapon Xiuhcoatl. He strikes down Coyolxauhqui first: he beheads her and her body breaks apart as it tumbles down the slope of Coatepec, "her arms, her legs, falling each in a different place." Then he turns on the four hundred and scatters or destroys them. The myth is at once a divine birth, a family murder, and the charter of Huitzilopochtli's supremacy: and, told of the Mexica's own patron god, a statement of the war that sustains the sun.