The Hangsman's Plague was an affliction that surfaced around the year 129 of the Fifth Age, and was rampant among sailors and fishermen in the Unquiet Ocean. It is spoken of in Postbag from the Hedge issue 43, in the journal of Duke Horacio's father, which recounts an expedition to Crandor, which was, at the time, a wealthy island-city of accomplished mages and adventurers, with a large market which attracted people from all over Gielinor, including Gnomes, Fremenniks and even Jogres.
"Hangsman's Plague" apparently afflicted the Monks on the vessel which Horacio and his father, the then duke of Lumbridge, were passengers of. According to the journal, which speaks briefly of it, the plague had horrific effects on the body, and the monks infected were "screaming all night with the hangsman's plague, keeping us and the crew awake." It is also reported that the young Horacio became ill at Crandor and ran a temperature, causing his father to rush back to the ship as he "feared the plague". Other than this there is no currently no known information about the Hangsman's Plague.