The Sewer set is an alternative Novice level set to the Caves set. It contains the following cards, only 3 out of 153 of which are replicated in the Caves set. You do not need the caves set to play the sewers set, however having both sets will give you more choice for your heroes and adventures.
4 Occupations, 6 weapons (3 weapons are the same as are in the Caves set), 15 weapon talents, 16 skill talents, 5 kit items, and 4 Outfits.
13 new Foes (the Lizardkin and more)
15 Locations, 11 barriers, 13 obstacles, 10 loot, 8 event cards, 13 discoveries, 12 reference cards. 8 Scenario cards.