|Hold it right there, outlaw!|
This article is littered with spoilers, so I reckon you might ought'a mosey on down the road if you don't want to read any plot details.
–Ayauhtéotl to John Marston
Ayauhtéotl is a major character featured in the Red Dead Redemption Undead Nightmare DLC pack. While omnipresent throughout the game, she is specifically encountered during the mission "A Civilized Man."
Ayauhtéotl was the Aztec being, force or Goddess who controlled mist, smoke and most particularly Crepuscular rays, the dark and light beams that appear under the right condition at sunset. She was also the Goddess of fame and glory. 
It appears that when Abraham Reyes stole a sacred artifact, in this case the Mask, she took human form to retrieve it. The Mask was cursed, reanimating the dead and a magical plague with them. It seems that she can disappear when she is in god-form. She has the ability to command the Death horse and lift the plague once her power is restored. Yet, in human form, it appears that she is mortal, and fears being killed by the Undead Reyes.
She first appears as a woman that is in both groups of survivors in Blackwater, then again with the Sasquatch Hunter. She also appears in the opening cutscene of the survivor mission "Dinner for Two" as she can be seen looking through the window. She later is seen in the opening cutscene of the "Missing Souls" mission. After John Marston meets Mother Superior, she again appears as a nun. Mother Superior tells him that the nun had told her that the curse was caused by Abraham Reyes, who disturbed an Aztec curse after retrieving something.
When John tracks Reyes down, he finds the zombie Reyes chasing this same woman around a table. After Reyes is dispatched, she shows John the Mask that Reyes took. Whilst taking him down to the crypt, her behavior becomes increasingly unusual, mentioning the zombies' anger and ancient curses while John notices that the Undead aren't attacking her. Much to John's surprise, she also quotes Jack on the story that he was reading: "Some people worshipped the moon, and the brave man has to kill everyone...". Upon reaching the Altar, she tells John to return the Mask and disappears as he does so. She says her name is Ayauhtéotl and a horse is waiting for him outside so he can return home. John Marston stood for a moment, confused of what happened. The horse is Death, one of the Four Horses of the Apocalypse.
- "Curious Tales from Blackwater"
- "Birth of the Conservation Movement"
- "Dinner for Two"
- "Missing Souls"
- "Mother Superior Blues"
- "A Civilized Man"
- Ayauhtéotl may be able to become invisible from some people's sight, or make them ignorant of her presence as in Blackwater she is seen with multiple survivors that don't seem to notice her at all. The Sasquatch Hunter seems to be the only one other than John that can see her, and claims to have rescued her from a Sasquatch.
- If John attempts to re-enter his house at Beecher's Hope before completing the game, a small cutscene will play that shows Marston decide against opening the door when he hears Ayauhtéot's voice asking: "Did you listen to a word I said?" This happens even if she has yet to be encountered. The house also has more wooden planks on the door than before.
- The player can kill her after her second appearance in "Curious Tales from Blackwater" by shooting just directly to the left or right of her head with a shotgun.
- Ayauhtéotl only appears with the family in the cutscene, and not in actual gameplay.
- Although Ayauhtéotl is an Aztec goddess, the mask appears to resemble the Were-Jaguar figures of the Olmec culture more closely than any Aztec artifacts.
- Her mask could possibly be the treasure that Basilio Aguirre Olmos de la Vargas talked about.
- She wears the same clothes as Millicent Waterbury.