All's Right With the World
Complete "On a Pale Horse"
This mission begins after completing all the Mother Superior missions in Mexico.
Ride to Escalera, to the music of the Kreeps' Bad Voodoo, where upon arrival you'll encounter a woman crying on the steps of the Reyes mansion. Go inside to discover a zombified Reyes attacking a familiar woman. You'll have no choice but to kill Reyes, after which the woman will tell you about the mystical powers of the mask and that it must be returned to it's altar. Follow her to the cave entrance near the bull fighting ring in town. Fight your way through the tunnels and place the mask on the altar.
The woman will vanish, but her voice tells you to return to your family. That's all I can say at this point without a major spoiler.
Needless to say upon completion of this story arc you'll be rewarded with Death (one of the four Horsed of the Apocolypse) and the achievement will pop.