Walkthrough for Missing Souls of Red Dead Redemption: Undead Nightmare.
1. Go to Fort Mercer under Rio Bravo to find a poster behind the entrance gate.
2. Take the Poster on the wall to start the mission.
3. Go to Pleasance House left of Cholla Springs marked with blue Missing Souls icon to find Millicent Waterbury.
4. Kill zombies around the house.
5. Go back to Fort Mercer marked with Brown Dot in the map.