Undead Nightmare is a zombie horror-themed addition to what Western action adventure video game?
Undead Nightmare: Undead Nightmare is an expansion pack to the 2010 video game "Red Dead Redemption". Undead Nightmare:  As a standalone episode, it adds a zombie horror-themed single-player campaign, two multiplayer modes, and cosmetic additions to the environments and characters of the open world Western action-adventure game. Red Dead Redemption: Red Dead Redemption is a Western action-adventure video game developed by Rockstar San Diego and published by Rockstar Games.
Red Dead Redemption