The Dreamlands Underworld Monster expansion consists of three monster types. It includes 4 Ghasts, 3 Leng Spiders, and 2 Gugs. They can be used on any map - i.e., not just the Dreamlands. You can use them on Yuggoth if you're of a mind, and they work fine. Their availabilty in the waking world is a sign of the bounds of reality breaking down as the Great Old Ones shatter natural laws on their return.