Go to the sewer gate, here and go down, walk south and be prepared to fight some Slimes. Go down another floor and fight more Slimes. Finally use the poison flask on the water, this will prompt to update your Quest Log.
Player: hi Padreia: Welcome to our humble guild, wanderer. May I be of any assistance to you? Player: mission Padreia: Oh Player, thank god you came to me. Last night, I had a vision about an upcoming plague here in Carlin. ... Padreia: It will originate from slimes that will swarm out of the sewers and infect every citizen with a deadly disease. Are you willing to help me saving Carlin? Player: yes. Padreia: I knew I could count on you. Take this high intensified vermin poison. In my vision I saw some kind of 'pool' where these slimes came from. ... Padreia: Pour the poison in the water to abandon the demise of Carlin. Tell me about your mission after you fulfilled your task. Player: bye Padreia: Farewell.
Player: hi Padreia: Welcome to our humble guild, wanderer. May I be of any assistance to you? Player: mission Padreia: You did it! Even if the fewest of the Carliners will ever know about that, you saved all their lives. Here, take this as a reward. Farewell! Player: bye Padreia: Farewell.