The Wailing Death is the magical plague that was crippling the city of Neverwinter at the start of the original campaign. The plague withdraws heat from its victims and puts them in great pain, eventually causing their death. Because of the plague's magical nature, it could not be cured via conventional means. Curing the plague and tracking down its source are central to the plot of the campaign.
The only known cure is a potion made from the essences of the Waterdhavian creatures—a lock of dryad's hair, yuan-ti blood, a cockatrice feather, and the brain of an intellect devourer. The PC collects these reagents by sorting out Neverwinter's affairs in chapter one of the original campaign, eventually ridding the city of the disease. At first the cult member Desther escaped with the cure, but the PC manages to recover it.
The plague was spread by cult members disguised as Helmites, in their "blessings". The reason for the plague was to both de-stabilize Neverwinter and to strengthen the cult's leader, Queen Morag. Every death caused by the plague channeled energy into Morag.