Rats are the embodiment of the plague, the physical interpretation of an intangible disease. It’s the element from history that we transformed to give it something more fantastic. they are both an obstacle for the player, a key narration element, and a triggering factor.

Background
It started with bites in the night... then the sickness spread.
FATHER THOMAS EXPLAINS THE INITIAL OUTBREAK OF THE PLAGUE
The rats have likely been the distributor of the plague ever since the Prima Macula first manifested, and have been traced as far back as Justinian's Plague in 541 AD, centuries before the events of 1348-49. The Château d'Ombrage, an ancient French fortress, was designed with defense against the swarms in mind with pitfalls and moveable braziers to flush the vermin into the holes where they cannot escape. At some point, however, they overran the nearby village in a single night, leaving it in ruins, as well as possibly doing the same to the castle, leaving it in its current state.
de Rune Legacy
In November 1348, when Amicia de Rune and her father Robert de Rune are in the forest near their home in France, the threat of the rats is already present (although unseen at first). While chasing a boar with her dog, Lion, Amicia loses sight of both for mere moments and is shocked to find the boar not only dead but stripped of its flesh. Shortly after, she finds Lion still barely alive and draped out of a hole. Shortly after, the unseen rats then dragged the poor beast into their hole to Amicia's and Robert's horror. Once the two return to the estate, a servant can be overheard talking about "bites" being reported in the nearby village, although this is dismissed as being an English plot. Later, when fleeing the Inquisition, many of the assailants begin to call for retreat as rats begin to emerge from the ground and attack them, allowing Amicia and her younger brother Hugo to escape.

The Strangers
Amicia and Hugo finally reach a nearby village where they discover that a plague has been unleashed upon the populace by rat bites. At this point, no one has realized that the rats are responsible, but they did witness a swarm invade the local church. In their grief over lost or infected loved ones and fear of the disease, the villagers lead by Conrad Malfort begin burning those they think responsible for bringing the vermin, including Amicia and her brother. They find shelter with an old woman named Clervie.

Retribution
The two hide in the town church with Father Thomas, who leads them to a hidden entrance through underground passageways. Thomas had discovered that it was the rats that had spread the disease, but it was too late. Here, the rats make their first visual appearance when Thomas attempts to find his fellow monk, Brother Morel. He is caught in the open, dark passage by rats and is quickly devoured. Using the rats' fear of torchlight, the pair makes their way through the rats' nest and escape.

The Apprentice
Amicia and Hugo go to find Laurentius, whose farm is already all but overrun by the swarm, which the Inquisition attempts to contain to a single area by slaughtering masses of pigs. A rat swarm surfacing within Laurentius' house accidentally knocks over a barrel of distilled alcohol onto an open flame, causing an explosion which burns down the house. Amicia, Hugo, and Laurentius' apprentice Lucas escape to the river, where the rats seemingly refuse to brave the water.

The Ravens' Spoils
The rats have infested the site of a recent, brutal battle between the French and English Army, where they gorge on the corpses of humans, horses, and prey upon any survivors. If any English soldiers catch onto the trio, Amicia can use her sling to destroy their lanterns, condemning them to a grisly death by the horde. Without light, their armor provides little defense against the hundreds of ravenous, hungry mouths. The rats were absent in the English camp during the daytime, although some of the soldiers can be heard discussing their hatred of the creatures. One soldier calls them "a divine plague." Thanks to all the English corpses devoured by the swarm, the English have had trouble obtaining a proper count of their losses against the French.

The Path Before Us
Amicia and company discover a number of means to help combat the rat swarms like Luminosa, which incinerates rats in a small radius and take advantage of some traps. Given their ancient design, Lucas and Hugo correctly speculate that the rats are a much older threat than they seem. Once outside, rat swarms that block the path are scared back into their holes by lightning, but only temporarily. Entering the Château d'Ombrage, the children manage to trap all the rats in the area in a pit, eliminating them as a threat in their new home. Amicia and her friends gain a brief respite from the rat hordes within the ancient castle.

In the Shadow of Ramparts
The Inquisition has begun evacuating the city surrounding the University when the Bite is found to be spreading there; however, Amicia discovers empty cages, indicating that the rats were unleashed on purpose. Amicia discovers new tools used to fend off the rats such as a swiveling light beam and Odoris, which temporarily attracts rats to the area where it's thrown. Due to the efforts taken to secure the University for the Grand Inquisitor Vitalis' arrival, rats are not seen in this area at all, and the defenses at the château have worked as well.

All That Remains
Amicia and Lucas return to the de Rune estate to find it having been turned into a nest, with the black filth piled everywhere and rats in such immense swarms they now have a way to overcome sources of light by forming small swirling masses of piled rats that hurl themselves forward and require being directly attacked with Ignitus to disband. Surprisingly the vermin have left many of the body of the estate residents, including Robert, untouched, and Hugo's room is left as it was. The two find Béatrice's lab and the rats seem to be actively fighting Lucas' efforts to make the cure, becoming more riled up and combative than ever before. The rats are, in fact, directly tied to the Prima Macula, and this plague outbreak is not the first to have occurred. According to a Roman fresco, this curse is cyclical and has been occurring for at least 800 years. Once Lucas succeeds in making the cure, the swarm falls completely calm and allow the two to leave.

Penance
The rats appear as an element of Amicia's nightmare, forcing her to wade through them, and she can only watch helplessly as Hugo is led away by Lord Nicholas while the rats swarm over her before the dream ends.

Blood Ties
When Hugo manages to rescue his mother, although only briefly, she shows him the ability to control the rats, at first using them to merely frighten away the guards. However, when Vitalis' men attack Béatrice he is forced to use them to ruthlessly slaughter the Inquisitors, fully awakening his abilities.

Remembrance
A month has passed, and the residents of the château prepare the defenses for nightfall, but are horrified when they are immediately overwhelmed by a swarm of rats surpassing any seen yet, so many that they overflow the pit. Lord Nicholas and Hugo arrive shortly after, and Hugo almost uses the rats to kill Amicia out of his anger for her lying about their mother being dead, but he forgives her. Instead, he uses the rats to attack Nicholas, who manages to fend off the swarm by setting his clothes and sword on fire. Once Amicia extinguishes the flames, the rats charge the Inquisitor one last time, dragging him screaming and kicking into the ground to never be seen again. The children then decide to use Hugo's power to end the Inquisition once and for all.

Coronation
Amicia, Hugo, Rodric, Lucas, and Mélie arrive in town to find the cathedral having burned turned into a massive disgusting nest. Using Hugo's horde to fight their way in, the find that Vitalis had let the congregation be slaughtered to feed his "angels,"

While small and of little threat alone, rats instead appear in massive swarms that blacken the ground with their approach. Their squeals fill the air and their wicked little eyes glow orange in the light of one's torch. When a swarm arrives, it can strip flesh from bone in less than a minute, devouring anything and everything they find. Man or child, human or animal, dead or alive: it is all merely food to satiate the frenzied swarm. Many of the rats seen are still drenched in blood and gore from their last meal.

Those that survive a rat attack are far from lucky, for these creatures carry the Bite. Anyone bitten or having come into contact with infected persons will likely contract this deadly, agonizing disease.

While the hordes of rats mainly make their way above ground, they hide underground in nests lined with filth and remains during the day and will often tunnel their way about to avoid light from fires or torches, emerging with a rumble to pour into the open world. Light represents their only real weakness, be it sunlight or man-made light such as torches, although they can also be distracted by food. However, they can group up and form twister-like, swirling mounds of rats to hurl themselves against light sources to extinguish them.

Rats generally weigh 8oz, range between 12-18 inches long, have dark fur, and have long, furless, worm-like tails. They scurry on four legs, have large ears, and keen noses able to smell food from a considerable distance. Hairless rats appear near the end of the game, having a distinctly white appearance compared to their usual brethren.

The rats provide a very similar game element to the Kryll swarms in Gears of War.

Both of them are small, fast creatures who hide underground during the day, and form immense swarms.

Both of them are threats in dark areas capable of stripping the flesh from anything unfortunate to be caught away from the light.

They are both deterred by light, be it sunlight or artificial light from fires.

Lastly, they can be controlled by special characters, the Kryll by General RAAM and the rats by Hugo and Vitalis post-threshold.

Rats are also a very dangerous game element in Dishonored, where they are also a flesh-eating, plague-bearing swarm threat which cannot be killed effectively, only avoided.

Rats that are caught in a new source of light and unable to pathfind away from it within a few seconds are seemingly incinerated even if just by the light of a torch.

Rat swarms are still witnessed today in India called mautam where they swarm and reproduce in large numbers in response to the seeding of bamboo plants; however, rather than eating humans and animals, they primarily damage crops and cause famine.

Rats to this day are associated with having spread the plague themselves; however, it was not the rats that spread the disease, but the fleas they carried. As rats died from the disease themselves, their fleas would leave their dead hosts to find living ones, thus infecting humans.