Weepers are plague-infected civilians who are aggressive and can spit corrosive bile. They are not technically 'bosses' but can be dangerous in groups. They are immune to sleep darts. For a lethal approach, a single sword strike or pistol shot is usually enough. For a non-lethal approach, you must avoid them entirely or use environmental hazards to incapacitate them without direct engagement. They are often found in plague-ridden districts and sewers. Be careful not to get too close.
