Rat poison is used in the Clock Tower quest. It can be found in the northwestern (white) section of the dungeon. Go down the ladder south of the clock tower, follow the path to the middle and take northwestern door near the white tile. Follow the path northeast and the Rat poison will be located in a dead end. It can also be used in the Ratcatchers quest. The rat poison is untradeable, and so once the Clock Tower and Ratcatchers quests are both complete, the item becomes useless.