On a Pale Horse

 is technically the final mission in the Red Dead Redemption Undead Nightmare DLC pack.

Description
It is not really a mission though, and it seems to be more the permanent state of the Undead Nightmare world upon completion of the game. There is no specific loading screen title card and the mission seems as if it's simply a follow-on from &quot;A Civilized Man&quot;, but when the player saves the game, the title given will be "".

Story
In the epilogue of the mission &quot;A Civilized Man&quot;, Seth Briars is seen stealing Ayauhtéotl's Mask from the Escalera Catacombs. An action that apparently takes place following the events of &quot;The Last Enemy That Shall Be Destroyed&quot; because it also causes John Marston to rise from his grave as well, though he does not become a mindless flesh-eater.

According to the in-game message:

"Marston was buried by his wife and son with a flask of water he seemed to treasure. They did not know of its special power. Now, he comes back to this earth with the body of an undead wretch, but the soul of a man. Help put him to rest by finishing his work on earth."

However, unless the player still has some game elements incomplete (such as ambient challenges or Missing Souls missions) there does not appear to be any other mission upon which Marston can embark that will 'put him to rest'. Seth and the Mask are nowhere to be found and the entrance to the catacombs has closed. Towns will still fall victim to undead outbreaks, but the outcome will be the same as when the player completed these tasks before this last mission.

Trivia

 * Like other missions in Red Dead Redemption, the name "On a Pale Horse" is a reference to a passage in the Bible. In this case, it is Revelation 6:8, what the exact quote is "behold the pale horse the man who sat on him was death and hell followed with him".
 * The mission's title no doubt also refers to the novel of the same name by Piers Anthony, the first in his 8-part series the Incarnations of Immortality. In the novel, a photographer named Zane attempts to commit suicide by shooting himself. When he is startled by the personification of Death itself, he panics and kills Death instead. As a result, Zane must take up the mantle of Death and be an arbiter of whether souls will go to Heaven, Hell or Purgatory.
 * The name "On a Pale Horse" could also be a reference to the horse, Death which was white in the book of revelations, that the player unlocks on completion of the game.

Achievements/Trophies
The following Trophies/Achievements can be acquired upon completing this mission: