[The book was a trap, don’cha know.]
So after we sacked the warehouse (with the sad, questionable execution of Serpentlady), it turns out they were one step ahead and have probably already spread goo via water supply in the sewers. But, having found the secret sewer entrance in the previously mentioned warehouse, we can narrow down the possible places for where the Serpent Guard could be hiding. Thing is, we need to talk to the Minister of Guilds to get a map of the sewers so we can find the holes in it.
But first! We should talk to the Arbiter General to see if he can help us with that (the getting the sewer map). Because we trust him and stuff.