Rats

Rats, specifically Pandyssian Bull Rats, are carriers of the rat plague, spreading the disease which infects the inhabitants of the city of Dunwall.

They can be encountered throughout Dunwall, both in swarms and on their own. Rats and swarms can also be found in Karnaca, though swarms are much less common. Rats are aggressive in high numbers, and large groups are more likely to attack Corvo Attano, Emily Kaldwin or Daud if they near them. While they are cowardly in smaller numbers, in swarms they can easily take down and consume a lone watchman in a matter of moments.

Rats congregate in dark places and wherever corpses can be found. Although they are deterred by bright lights and large gatherings of people, they will notice if the former fails, and will investigate any newly-available areas.

Combat
The noise and spread of a pistol is a good way to quickly disperse a crowd of rats, killing some of them in the process. However, it is ill-advised to stay nearby for too long, as the dead rats will respawn with any remaining rats and will swarm at the protagonist once more. An easy but time-consuming way to get rid of rats is by standing on higher ground, such as a chair or table, and hacking at the swarm of rats below. They will be unable to attack, while at the same time make easy targets for sword attacks.

Any thrown object can be used to kill rats, including bottles and bricks.
 * Grenades and Howling Bolts are effective at getting rid of large swarms of rats, but can easily attract other enemies.
 * Chokedust kills the rats without hurting the people being attacked by them.
 * Windblast can be used to dispel rats, killing enough to scare them off even if it is not upgraded.
 * Pull will instantly kill the whole swarm, not just the rat it was used on.

Supernatural Abilities
Corvo has two supernatural abilities that take advantage of these animals: Possession and Devouring Swarm, with the latter allowing Corvo to summon a group of rats for the purpose of attack. Accompanying these are several bone charms that provide bonuses when interacting with individual rats or rat swarms.

Billie Lurk has an artifact that grants her an ability called Rat Whispers. This allows her to understand rat speech, gleaning important information from them. The rats also refer to Billie as "one of them" and "friend".

Dishonored

 * Albinos - Increased chance of white rats.
 * Carrion Killer - Killing rats grants some adrenaline.
 * Rat Scent - Rats attack only when approached in close proximity.
 * Welcoming Host - Possession lasts 30 seconds on white rats (normally 20).
 * Gutter Feast - Eating white rats restores 5% mana.
 * White Rat Friend - White rats are not hostile.

The Knife of Dunwall and The Brigmore Witches

 * Carrion Killer - Killing rats grants some adrenaline.

Dishonored 2

 * Accommodating Host - Animal Possession lasts slightly longer.
 * Albinos - Rat swarms include more white rats.
 * Blood Sacrifice - Killing bloodflies or rats restores Health.
 * Carrion Killer - Killing rats or bloodflies boosts Adrenaline.
 * Gutter Feast - White rats can be eaten for Mana.
 * Spiritual Sacrifice - Killing bloodflies or rats restores Mana.

Trivia

 * In comparison to native rats, Pandyssian Bull Rats are larger, have coarser fur, and possibly differ in intelligence.
 * 'Rat Burning' is a practice whereupon the participant (typically a watchman, such as Sergeant Heyburn) will toss a live rat into a wall of light.
 * Captain Geoff Curnow has forbidden his men from taking part in this practice. Despite their commanding officer's orders, Sergeant Heyburn and his fellow watchmen still take part in 'Rat Burning'.
 * A "Bull Rat Fetus" can be found in a jar in the upstairs lab area of Anton Sokolov's Safehouse as well as the Offices of Doctor Galvani and in the Drapers Ward Textile Mill.
 * Rats will never attack Granny Rags, even when summoned with Devouring Swarm. Instead, Granny Rags will thank Corvo for summoning "more of her little pretties".
 * Due to the latter's advanced stage of plague infection, rats will ignore weepers; however, they will devour the corpses of those who died of the plague.
 * If unconscious or dead bodies are not lying on a climbable object or in a dumpster, a swarm of rats may spawn and devour the bodies. Having a high chaos or a large concentration of bodies makes this more likely.
 * Unconscious bodies eaten by spontaneously spawned rats count as killed by the protagonist and count against the respective achievements.
 * Many maps feature pre-spawned rat swarms. Unconscious bodies eaten by such rats do not count as kills against the player and do not count against the achievements.
 * Unused Heart lines for rats can be found in Dishonored's game files.
 * Rats and bloodflies are natural enemies: rats eat bloodfly larvae while bloodflies lay eggs on rats.