Devouring Swarm

Devouring Swarm is the supernatural ability to summon forth a swarm of ravenous rats from the Void as a means of offense.

Corvo Attano and Granny Rags can use this ability as a result of the mark bestowed upon them by the Outsider.

Rats summoned by Corvo do not fall under his control, although he can possess any one of them if he chooses. As they have a mind of their own, they may attack any non-player character, disposing of them in an "especially grisly manner." They will completely consume the bodies of Corvo's victims if given enough time, as would any swarm of naturally occurring rats.

Because of the nature of the power, it is useless in a non-lethal run. However, it proves extremely useful when Corvo is not concerned with being non-lethal, but wishes to remain unseen. From a high point or hiding spot, Corvo can dispatch hostiles or remove corpses by calling the rats. As this power does not require Corvo to show himself to the enemy, it is one of the few offensive abilities that does not lead to automatic detection (although any remaining hostiles in the area will be on high alert afterward). In addition, even in the midst of combat, Devouring Swarm proves useful against mobs of enemies.

When Granny Rags uses Devouring Swarm, unlike Corvo, she seems to have a degree of control over the rats--they will not attack her, and she will thank Corvo for summoning more of her "little pretties" if he uses the ability against her.

Like naturally occuring rats, the  summoned rats will not attack weepers.

Ability tiers

Tier I - (3 runes)

Summons a swarm of rats which attacks and can kill and eat foes.

Tier II - (4 runes)

The swarm's attacks do more damage, and it eats foes faster.

Gallery

A devouring swarm of rats attacks a thug.

A crowd of rats attacking a pair of City Watch Officers.

The Lonely Rat Boy uses Devouring Swarm in the second chapter of The Tales from Dunwall, The Hands that Feeds.