This is a thread for all the things in Skyrim that exist in the game's code, but cannot be found without using the console or hacking.

There is an in-game faction with no members called the Agents of The Black Marsh. It's relationships and editor ID suggest to me it has to do with the quest Unfathomable Depths. The questgiver and one of her deceased companions were Argonians, so presumably they were originally going to be in the employ of the government of Argonia.

As well, there is a war axe called the Briarheart Geis, which does extra damage towards Nords.

Any other examples of similar unused stuff you can name?