Mictlan was the Aztec underworld, ruled over by its Lord and Lady. It was a gloomy place, reached by the dead only after wandering for four years beneath the earth, accompanied by a "soul-companion", a dog which was customarily cremated with the corpse.
Aztec myth tells how the god Quetzalcoatl journeyed to Mictlan in the Fifth Sun in order to restore humankind to life from the bones of those who had lived in previous eras. For bones are like seeds: everything that dies goes into the earth, and from the earth new life is born in the sacred cycle of existence.
Quetzalcoatl approached the Lord of Mictlan, where he sat on his throne surrounded by spiders and owls. "I've come for the bones, the precious bones, the jade bones," said Quetzalcoatl. "Can I have them in order to populate the earth?"
Only unwillingly did the Lord of Mictlan gives his assent. "You may take away that which I guard so carefully on one condition - that you parade four times around my throne blowing on this trumpet." And he handed Quetzalcoatl a conch shell that had no finger holes. But worms bored the fingerholes, and bees flew inside to make a sound.
Even so, Quetzalcoatl knew that he'd better move quickly to take the bones and leave. And sure enough the Lord of Mictlan gave orders that the bones be recovered. Quetzalcoatl thought of a trick. "Tell the Lord I'll leave the bones behind," he said to his nahual, his spirit twin. Accordingly the nahual, looking just like Quetzalcoatl himself, assured the Lord of Mictlan that the bones would be left. Meanwhile Quetzalcoatl began to run. Unfortunately, the Lord of Mictlan ordered that a pit be dug in the fleeing god's path, and sure enough he fell into it, having been startled by a covey of quail. Those bones that weren't already shattered were pecked at by the quail. Which is why humans come in all sizes.
"This has not worked out well," said Quetzalcoatl to his spirit twin.
"What must be must be," replied the nahual. And so Quetzalcoatl scooped up the bones and, once safely beyond the dead land, ground them up in a bowl. Together with other gods, he sprinkled them with his own blood, restoring them to life. And thus humankind was born from the pennance of the gods themselves.
[Based on the Myth of the Suns and the Toltec-Chichimec Origins of the Mexica People., as translated by Willard Gingerich, in The Flayed God. by Roberta H. and Peter T. Markman, published by HarperSanFrancisco. Available in paperback and highly recommended.]