I almost agree with Ceaen that it could be eerie fog but the funny thing is, I checked. It didn't give the same effect as the avi. I have a hunch that it could be already part of the background used by the user. Wish I knew what it was.
Is one of the Venator items backgrounds, they layer behind with certain other backgrounds (in this case the Timeless Swamplands Circe + Well Done Trick or Meat ) to give that fog feeling
so it's not a item giving that feeling but 3 backgrounds layered