The phrase "sewer dungeon" in the lines suggests that the brook (a small stream of water) has been confined or trapped in a dark, underground place.
The use of the words "sewer" and "dungeon" implies a grim and unpleasant environment. Sewers are typically associated with drainage systems and are not places where a natural brook would be expected to flow.
The word "dungeon" further emphasizes a sense of confinement and imprisonment, adding a negative and oppressive tone to the description of the brook's surroundings. Overall, the phrase suggests a stark contrast between the natural beauty of a brook and the harsh, confined conditions it now faces.