Undead Nightmare is a modified expansion of the 2010 video game Red Dead Redemption, an open world Western action-adventure in which the former outlaw John Marston is coerced by the federal government to capture former members of his gang in exchange for his family's freedom. Undead Nightmare is an alternate timeline expansion that adds a new full-length story in a zombie horror theme and reworks the base game's environment to appear dark and spooky. As corpses begin to rise from the dead to become flesh-eating zombies, some of the living go into hiding and others become hostile. John seeks to find the plague's cause and its cure for his infected wife, Abigail, and their infected son, Jack. The game's story lasts about six hours and includes new cutscenes, voice acting, and music. It is accessed separately from the main game menu with its own save files. It does not share play progress and items from the main game, nor does it require the base game in any form. 
As in Red Dead Redemption, the player can diverge from the main story on side quests: requests from strangers, checklist challenges, and other digressions. These missions are similar in format to those of the original but their objectives are replaced to fit the zombie horror theme. For example, the player hunts for the missing family members of survivors instead of fugitives or empties graveyards instead of gang hideouts. John is called to liberate towns overrun by the undead, which unlocks ammo, new places to stay, quests, and the assistance of the living townspeople. After John's assistance, the towns remain free of zombies for several in-game days, but will eventually require the player's assistance when zombies return once again. The player decides whether to share ammunition with the townsfolk: ammunition serves as a currency in the game since the shops have closed. The player has fewer correctives for immorality, such as helping or killing other survivors, given the dire circumstances. The game puts a greater emphasis on hyperbolic fun. 
The expansion's zombie enemies are similar to that of other games. "Walker" zombies are slow from afar but run when they near the character. Fat zombies attempt to knock the player-character down and others spew green venom. John has new weapons, including holy water, bait for the undead, and a blunderbuss that uses zombie parts as ammunition. The player can use a combination of these skills to defeat hordes of reanimated humans, who stop only once their heads explode. For example, John can bait a group to an area to set off a bomb, use the time-slowing Dead Eye mode to shoot each in the head, or eliminate enemies from a roof because zombies cannot climb. Other non-playable characters who contract the infection turn into the undead in real-time and are likely to attack the player. 
Undead Nightmare removes the traditional fast travel features of the base game, and this encourages the player to use settlements to store their caravan. John rides undead horses in Undead Nightmare, which he can summon by whistling. Four mythical horses—the four horses of the apocalypse—with faster abilities are scattered throughout the game for the player to find, tame, and ride. Other mythical creatures to hunt include Sasquatch, El Chupacabra, and a unicorn.[4] Also included are two new outfits for John, other cosmetic options for weapons and player horses ("mounts"), and achievements.