Go to the Decrepit Ferry. Search for the reason the Scourge are there.
[player], we've discovered a new hive for the Scourge: the Decrepit Ferry. The ferry lies east of the Sepulcher, and is heavily patrolled by rot hides and other undead.
There must be a reason for the Scourge's occupation of the area. Go to the Decrepit Ferry and find out why the Scourge are there.