A standalone expansion pack to the 2010 open world Western game Red Dead Redemption, it adds a non-canonical zombie horror–themed single-player campaign, two multiplayer modes, and cosmetic additions to the environments and characters.

They found the Red Dead Redemption universe and its American countryside to fit the cinematic heritage of the horror film genre.

Undead Nightmare is a modified expansion of the 2010 video game Red Dead Redemption,[3] 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.

[4] 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.

[7] As in Red Dead Redemption, the player can diverge from the main story on side quests: requests from strangers, checklist challenges, and other digressions.

John is called to liberate towns overrun by the undead, which unlocks ammo, new places to stay,[3] quests, and the assistance of the living townspeople.

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,[3] or eliminate enemies from a roof because zombies cannot climb.

[3] Undead Nightmare removes the traditional fast travel features of the base game, and this encourages the player to use settlements to store their caravan.

[3] Shortly after being reunited with his family, John Marston (Rob Wiethoff) is trying to lead a peaceful life with his wife Abigail (Sophia Marzocchi), son Jack (Josh Blaylock), and friend Uncle (Spider Madison) on their ranch at Beecher's Hope.

He runs into one of his former allies, Professor Harold MacDougal (Joe Ochman), who theorizes that a virus has caused the dead to come back to life.

After MacDougal is killed by an undead Nastas, John encounters other survivors and helps clear Blackwater and the nearby cemetery of the undead, before learning that another two of his former allies, con artist Nigel West Dickens (Don Creech) and treasure hunter Seth Briars (Kevin Glikmann), are supposedly responsible for the outbreak.

He meets with both men, who deny any involvement, although Seth voices his suspicions that the Aztecs had something to do with the entire ordeal and tells John to travel to Mexico.

He encounters a group of nuns led by Mother Superior Calderón (Irene DeBari), who informs him that a woman told her Abraham Reyes, whom John had previously helped take over Nuevo Paraíso, is responsible for the outbreak.

[10] Rockstar found a match in the Red Dead Redemption universe for their long-standing aim to create a zombie game.

In lieu of making a new intellectual property for such a game, Rockstar instead wanted players to see how a zombie outbreak affected a world already familiar to them.

"I don't think we ever saw the phrase 'what this game is missing is the supernatural'", Houser recollected on the team's feedback and their interest in doing something that would not be expected by fans yet enjoyed nonetheless.

"[10] Hip hop musician Oh No made drum tracks for the expansion as preparation for his work on Grand Theft Auto V.[11] Rockstar released Undead Nightmare on October 26, 2010.

Rockstar's Dan Houser said that the team was satisfied with their results and how the Red Dead Redemption world and zombie theme supplemented each other's context and depth.

[12] The release compiles the Undead Nightmare campaign with two smaller Red Dead Redemption download packs: "Legends and Killers" and "Liars and Cheats".

[8] Dan Whitehead from Eurogamer credited Undead Nightmare's nuanced treatment of John Marston as proof of him being "one of gaming's great characters".

[6] As a continuation, "Red Dead Redemption's elegiac tone knits incredibly well with [its expansion's] arch Gothic horror", wrote Whitehead.

[3][5][17] Whitehead added that Rockstar's ability to use the "most played-out and over-exposed cultural meme" in their "epic Western" without the product appearing silly or gimmicky was a testament to their design acumen.

While Whitehead wrote felt positively of the treatment of the main character, he considered the expansion's mission design lackluster and complained of too many quests in which the player fetches items for other people and traveling between points only to initiate cutscenes.

The reviewer compared the game's reinvigoration to that of the download content created for Rockstar's Grand Theft Auto IV.

While the idea of zombie animals scared Whitehead, he was both relieved and disappointed to find that bears and cougars, like the humans, died with a single bullet.

[21] Red Dead Redemption's PlayStation 4, Nintendo Switch, and Windows versions were praised for the inclusion of Undead Nightmare, though some critics lamented the US$49.99 price due to lack of significant enhancements and omission of multiplayer.

[25][26][27] Kotaku wrote of its originality, despite the trite zombie theme, and added that the expansion functioned best when Rockstar used the game's pretense to exercise their sense of humor, with undead versions of normal animals, and new mythical creatures.

[28] Hardcore Gamer cited the expansion as an example of downloadable content that would have never fit in the main game but instead let the developers explore other ideas.