The Egg of Coot recently was found in a Skaven Bedlam Pod by members of the Ironcloaks in the ruins of the Black Dogg Inn. However, before it could be processed for evidence it was "misplaced".
It's currently unknown if the Egg has invaded this plane or was a product of the the Bedlam Pod.
It seems too coincidental that the Egg arrived mere days before Essex College performed the King in Yellow. Further, the Egg has been linked with some of the survivors of said performance.
The Egg is nearly as large as a halfling in covered in scales. It seems to possess telepathy and refers to those under it's control as Thralls. A Thrall, thus far, is easy to detect due to their scleras and corneas taking on the same scaly texture as the Egg.