Rat Poison is a written note found in The Knife of Dunwall.
We need to rid the sewer of the rat infestation. Make sure you set this crate of potted meat aside so we can lace it with rat poison. After it is poisoned, take it down to the docks so we can get it into the sewers below.
It can be found in a side building early in the mission A Captain of Industry, near several crates of potted meat.