Mask

 is an item found in Red Dead Redemption. It appears to be the cause of the Undead Plague in Undead Nightmare, after it was removed from its place by Abraham Reyes. After it is returned to its place by John Marston, the world reverts back to its original state. However, it is stolen by Seth Briars a few months later, leading to a new undead outbreak.

Characteristics
It looks a bit like an aztec mask, and is made out of a greeny stone.

Locations
It is found in the catacombs below Escalera