The Disturbed.

There's a shapeshifting nightmare out there: concealed a creature with unsettling intelligence and changing forms. Before it is seen, its presence is sensed through far-off murmurs, ephemeral shadows, minute alterations, and the oppressive feeling of being observed. Chase and Decay are two of its forms that can be eliminated. However, you will need to locate and kill their heart in order to destroy them. And you will suffer under the Decaying Curse as long as Decay persists in your world. It does more than just hunt. It manipulates your thoughts.