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For those unaware, the "Zeniverse" is a hypothetical shared universe that contains the various series owned by ZeniMax Media and the companies they control/contract. This includes Rage, Doom, Wolfenstein, Fallout, and The Elder Scrolls (among others). Since for a number of reasons, there's no way that all these series can possibly follow a linear timeline, I'm taking a page out of Nintendo's book by proposing that alternate timelines are involved.

Timeline one: Classic Era (BJ kills Hitler)

Spear of Destiny-->Nocturnal Missions-->Wolfenstein 3D-->Commander Keen-->Fallout: Brotherhood of Steel-->Fallout Tactics-->Doom (1993)-->Doom II: Hell on Earth-->Final Doom-->Doom 64-->Quake

Timeline two: The New Order Era (Nazis win WWII)

Spear of Destiny-->Nocturnal Missions-->Return to Castle Wolfenstein-->Wolfenstein (2009)-->Ultimate Challenge-->Wolfenstein: The Old Blood-->Wolfenstein: The New Order-->Quake II-->Rage-->Doom 3-->Resurrection of Evil

Timeline three: Wasteland Era (Hitler kills himself)

Spear of Destiny-->Nocturnal Missions-->Return to Danger-->Doom (2016)-->Fallout-->Fallout 2-->Fallout 3-->Fallout: New Vegas-->Fallout 4


So as you can tell, the three timelines at first start out identically, but change after B.J. Blazkowicz defeats General Fettgesicht at the end of Wolfenstein 3D's Nocturnal Missions. In one, B.J. is captured by a vengeful Hans Grosse and brought to Castle Wolfenstein. In another, he is captured while attempting to raid Castle Wolfenstein. And then there's the third, where the Spear of Destiny is recaptured by the Nazis and B.J. has to recover it again.

In timeline one, Hitler is killed by B.J. and World War II ends earlier. In timeline two, the Nazis win the war, thanks to knowledge of future technology given to them by the mysterious demon Abigor. In timeline three, World War II ends just as it does it in real-life: Hitler commits suicide, and the Allies drop atomic bombs on Nagasaki and Hiroshima.

In timelines one and three, technology across the world is improved, until resources begin to dwindle and a nuclear war follows in 2077. Though similar, the aftermath of the war in these two timelines differ significantly. In timeline two, the Nazi government is eventually abolished, but mankind is nearly wiped-out by an asteroid that collides with Earth in 2029.

Union Aerospace Corporation

There is one constant in all three timelines, regardless of how World War II ended: The Union Aerospace Corporation (UAC). Even in spite of an apocalypse in each timeline, they have managed to build their own bases and maintain a population on other planets. In the first timeline, they have bases on not only Earth and Mars, but other planets and moons, including Mars' moons Phobos and Deimos, and Jupiter's moon Io. The UAC also has a presence in the second and third timelines, but they don't seem to quite as vast by the time they make contact with Hell: The only known bases appear to be on Earth and Mars.

Since the UAC hasn't been mentioned (yet) in any of the Fallout games (which all take place in the United States), I'm assuming that either UAC was a secret organization, or they originated from Europe, Canada, Australia, or another part of the world that would prominently feature English-speaking people.

Since there's no word on how the Great War affected Australia, it's highly possible that they were isolated enough to not only escape a direct attack from either the United States or China and maintain a population that could expand themselves space-wards.

Canada's another possibility, since the United States annexed Canada before the Great War, presumably for their resources. Perhaps this would also allow American influence to spread to certain Canadian organizations, which would explain why Vault-Tec's logo appears in the 2016 Doom reboot. Another hint that UAC might be connected to Canada is that the settlement of Ronto (which has been speculated to be the post-War version of the Canadian city Toronto) is said to have a military force that rivals that of the Pitt. In the Doom series, the UAC is known for its accomplishments in the development of military-grade firepower, and it's reasonable to assume that UAC's weapons have been sold to its citizens in whatever country they came from.

There's also a picture of a Canadian citizen being confronted by American soldiers in the first Fallout game's intro. The Canadian wears combat armor similar to that worn not only throughout the Fallout series, but also by UAC marines in the Doom series.

The Elder Scrolls

Now where does The Elder Scrolls fit into this whole thing? Well, here's a video that could better summarize things better than me:

For those unable to watch the video, or haven't the time to do so, I'll take the liberty of summing it up: the "Hell" depicted in the Doom series is in fact one of the countless planes of Oblivion, which has been linked--presumably by accident--to the dimension containing Earth and Mars. This plane of Oblivion would likely be one rarely seen by the people of Mundus. Explaining why we don't really see any of Doom's demons/Daedra in Tamriel, etc.

Going with this video's theory, this could mean that the demon Abigor from Ultimate Challenge is an underling of one of the Daedric Princes (likely Hermaeus Mora, whose sphere includes forbidden knowledge), or perhaps even a Daedric Prince himself. He may even be the mysterious "Demonic Voice" heard prominently throughout the 2016 Doom.

In Wolfenstein: The New Order, there is an ancient order known as the "Da'At Yichud", which has harnessed advanced technology that seems alien compared to the rest of the world. Not only that, but their tech appears to resemble that of the Dwemer (Dwarves) from The Elder Scrolls. In the third game, Morrowind, it's been established that the Dwemer had disappeared long ago, except for one: Yagrum Bagarn, who was Master Crafter of the Dwemer from the First Era. Lore states that Yagrum didn't disappear like the others because he was away in another dimension, though it's never specified where this dimension was, nor what he did while he was there. Perhaps Yagrum visited the Earth/Mars dimension thousands of years ago, met the people of Da'At Yichud, and helped them build all their wonderous tools and inventions before he returned to Nirn.

Also, since The Elder Scrolls Online tends to have a number of huge differences that deviate it from The Elder Scrolls lore, I propose that it, along with the mobile version of TES IV: Oblivion, occurs in a different timeline from the main installments. It is likely that it would contain the version of Hell, as well as other dimensions encountered in the "New Order Era" timeline, while the "Classic Era" and "Wasteland Era" timelines would intersect with the main TES series and pre-Online spin-offs (Battlespire, Redguard, and the Travels games).