A Muck Mudlet  

These spawn during the mud ring. They come from "A Sludge Lurker" in pack of 10, but can perfectly be Crowd controlled.

A Sludge Lurker has to be brought down enough to make these spawn. As far as i've seen, depop occurs around 30% but it changed with each mob of this kind (by this i mean that one of the Sludge Lurker" didn't even depop to turn into these).It happens that the mobs spawn in the wall which screws the ring and bring it to an end.