Pus are pieces of unexploded wights that can be thrown and which cause paralysis and/or damage if they explode. Used by ghols as a long-range weapon, pus can be picked up by ghols and thrown at enemy units.
There are two kinds of pus packets, small and large. Small pus causes paralysis, large pus explodes causing damage, and causes paralysis.
Some maps have pus lying around on it, on most maps the only way to get pus is to have a Journeyman or Heron Guard heal a wight.