Ratuora

If you frequent the Taverns of Aurora, especially those in the Outer District, you will eventually hear the tale of Ratuora.
Deep beneath Aurora, below the Sewer system of the Inner District lies a city of Ratmen, known as Ratuora.
It is said that the ratmen use the sewers as hunting grounds for food, and at night they sneak into Aurora, snatching any unsuspecting victim they can, and dragging them all the way through the sewer, down into Ratuora to be eaten.
According to the stories, ratmen are intelligent beings, about the same size as humans with the body of a rat. They walk on their hind legs and use their clawed hands to kill their victims.

"I stepped outside in the alley to take a piss, you know, as we usually do.
Suddenly, I heard something behind me and turned around. There it was! Looming over me. Its eyes shining yellow, baring its huge fangs at me. Its warm stinky breath hitting my face. It stood on two legs swinging its clawed hands at me. Was it not for my quick wit, I wouldn't have thought of throwing my mug of ale at it, sending it screaming back to the sewers."

"But Seamus, you never bring your ale outside. The mugs are not supposed to leave the tavern!"

— Excerpt from a tavern conversation

Summary

The tale of Ratuora is a large unsubstantiated story. Menkh Quill a well-renowned storyteller, included the story in his book, Myths, and Tales from my travels in Erekiath, which made the story common knowledge across the country. Menkh Quill explained that he heard many versions of the myth from the taverns throughout the Outer District of Aurora, and eventually settled on recording the version with the most detailed accounts of the ratmen.
Both The Council of Elders and The City Council have dismissed these claims as nothing but drunkards tales, and bad excuses to their wifes for coming home late from the tavern.

Pay no heed to these drunken tales. Take a look around you. Does this beautiful city look like a place that has intelligent vermin living in the sewers? Besides, ours are the most well-kept sewers you'll find!

— Lucius Croft of The Council of Elders in Aurora

Among the poor and the homeless, parents use the story as a cautionary tale, to keep the kids away from the Shit Gutter, a large gutter running through The Outer District, carrying waste from The Inner District's sewer system.

Historical Basis

Since there's detailed recordings of the entire history of Aurora, dating back to the day the of its construction, it is highly doubtful that Ratuora has ever existed, as there are no mentions of anything remotely related to an underground rat city.
Another theory is that the Ratuora wasn't uncovered until the Sewer system was dug out by the dwarves from The Snowy Peaks. Since there were a few accidents during the excavation that lead to the deaths of two dwarves, some believed that it could be related to Ratuora.

Spread

It started out as being drunken tales across taverns in Aurora, but after Menkh Quill put the story in his book, Myths, and Tales from my travels in Erekiath, it became widespread knowledge across the country, which was probably what turned a small scary story from a tavern, into a conspiracy theory.