The Aztec have a historical reputation for human sacrifice, a practice common throughout the region. While both the Spanish and the Aztec suggested that the Aztec had taken human sacrifice to unprecedented levels, recent evaluation has called that into question.
In the world known to the Moorish slave Estevánico, the Aztec had been conquered by Spain over a decade prior to his discovery of his fantastic eyewear. Esperanza/Amal confirmed this, and that at least part of the conquest's success was do to the fact that Hernán Cortés had a device similar to Estevánico's eyewear. Esperanza/Amal surprised Estevánico further by telling him that Montezuma himself had given the device to Cortés.
Esperanza/Amal also warned Estevánico that the Aztec had learned much in 700 years. Estevánico did not fully grasp what this warning meant until, while residing in Mexico City, Estevánico was pursued by Aztecs who spoke a language that sounded faintly like Nahuatl.
What Estevánico never quite knew, and what Esperanza could not tell him, was that while in some timelines, the Aztec did fall, in at least one, they defeated the Spanish, and built a formidable empire. In the 23rd Century, the Aztec were locked in a battle with their rival, the Inca, also a formidable empire, and that their fight crossed back across both timelines and time itself.
Aztec in "The Man who Came Late"Edit
When Holger told Alianora and her family of his adventures in the many worlds and timelines he had visited, he mentioned a near-fatal encounter with "feathered demons, or pagan gods" who ate hearts and drunk blood. This story made Alianora shiver.
An earlier reference in Poul Anderson's A Midsummer Tempest confirms that these were indeed Aztec gods.