Her offspring , Huitzilopochtli , learned of this plan while still in the womb , and before it was put into action , sprang from his mother 's womb fully grown and fully armed and killed his sister Coyolxauhqui , together with many of his 400 brothers and sisters . Her offspring , Huitzilopochtli , learned of this plan while still in the womb , and before it was put into action , sprang from his mother 's womb fully grown and fully armed . He then killed his sister Coyolxauhqui and many of his 400 brothers .