Spoilers, obviously. John Marston fixes the zombie problem until Seth ruins everything. Maybe he wanted his zombie lady back. Now John himself is a zombie and will not rest until he finds the Unicorn! Although the game says that John has unfinished business, there are no other story missions to the DLC. Unless Rockstar decides to develop the zombie John further by adding more zombie DLC, this is the last we will see of him.\r
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Ayautheotl is the Aztec goddess of mist, vanity and fame. She was also the rays that would appear before sunset. They were bright and beautiful, but the sun would soon go down and darkness fall. Aztec gods with tl at the end of their names are the most powerful Gods, powerful beyond human grasp. The Aztec thought that fame was like a mist, seeping out of a person and distorting everything around.\r
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The DLC takes place when John reunites with his family, but there are several inconsistencies with the main game. Most notably, Uncle dies later in the main game than he does here. You could argue that bringing the mask back didnt just unzombifie, but also un-shot-in-the-face-with-a-shotgun everybody, meaning Uncle was brought back to life and then shot in the main game. However, I think its safer to just consider this as not canon, like an alternate universe, similar to a Treehouse of Horror episode on The Simpsons.