The Yuggoth map is quite unusual. It adds three new Areas with special powers.
The Slime Sea
The faction which controls the Slime Sea overlook is allowed to summon the useful Slime Mold monsters, which are a good mid-level monster. The problem is that they obey anyone who controls the overlook. So if you have summoned up a batch of Slime Molds, and then another person captures the overlook from you, they take control!
The faction which controls the Laboratory is able to use it to transform his Cultists into Brain Cylinders. Fans who have read The Whisperer in Darkness or seen the eponymous film know what I'm talking about. The cylinders still produce Power like a normal Cultist, but cannot move on their own (they have to be carried). They also cannot fight or be selected to take a hit in combat. Instead, when a Brain Cylinder is "dropped", the other side can capture it. Now they get the Power from it.
The Green Pyramid
The Green Pyramid contains the mighty Brain of Yuggoth, which boosts your faction in many useful ways. However, each Cleanup Phase, the Green Pyramid's controller has to check to see if the Watcher of the Green Pyramid has been awakened! The Watcher is a particularly loathsome Great Old One [later reclassified as a Terror] who is the enemy of every faction, and which, after destroying all units at the Green Pyramid, moves on a set path around the map wreaking destruction everywhere. It has its own unique figure.