The party first encountered the Rot in a forest outside of Muretsk and stopped Gogloth, a corrupted druid, from spreading it any further. Unfortunately, that would not be the last they would see of Gogloth, as he was behind the disappearances of several parishioners in Ordzina, where they were resting. The party tracked him down and killed him once again... only to find another clone further into the ruins they were exploring. In a brief moment of clarity, one of the clones admitted that there was a terrible being pulling the strings, but the being reasserted control before he could reveal its location.
The party did not encounter the Rot (or Gogloth) again until almost two months later, when their skirmish at the Red Hand camp attracted the attention of several corrupted creatures in the area. Since then, the Rot's thralls have been continuously harassing the party.
A local eladrin informed the party that these corrupted creatures began spawning shortly after a drow matching Gogloth's appearance entered the local grove with a small group. Ever since then, the grove has been inaccessible, and a sickness has been creeping through the Shimmering Woods.
The party investigated the grove and successfully put an end to Gogloth's influence on it, though the druid had already done some serious damage — both to the surrounding forest and to nearby towns. Tyrion, however, came away armed with cryptic images which may someday lead him to the source of the Rot.
But they'll need to find allies willing to fight alongside them, as the ancient evil is unlikely to go down easily...