|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.
It appears to be the cause of the Undead Plague in Undead Nightmare, after it was removed from the catacombs beneath Escalera by Abraham Reyes. After it is returned by John Marston, the world reverts to its original state. However, it is stolen by Seth Briars sometime after 1911 which leads to a new Undead outbreak, resurrecting Zombie Marston.
The mask itself is made of stone, possibly jade, green in color, and appears to be an ancient Aztec design.
Abraham Reyes finds this ancient Aztec mask deep within the catacombs under the mansion in Escalera (his current residence). John Marston stumbles upon the Mask when he enters Reyes' home during the last mission of Undead Nightmare. It is here that Ayauhtéotl entices him to open a case that is lying on a table in front of him, which holds the mask.