From the introduction:
The bloody rites of the Aztecs have their roots in the bloodthirsty predations of a Great Spirit known as the Crimson God. He demanded only one thing from those who worshipped him: human sacrifices.
Another American god, Quetzalcoatl, objected to these practices and spent generations maneuvering the Crimson God into a corner. The winged serpent forced his adversary to make an oath: “I will never again do harm, or cause another to do harm, to the creatures that walk the land, soar through the skies, or swim in the lakes, oceans, or rivers.” This is the Curse of the Crimson God.