In the years before the advent of arcanotechnology and dimensional fatigue events, teratology was the study of abnormalities and biological deformations. In the wake of the second renaissance, it became the study of exomorphs, extradimensional entities, and most prominently, teratomorphs, more commonly and vulgarly known as kaiju.
Also known as Kaiju Spawn, Magna-parasites, angel-spawn, sin spawn, and a variety of other terms
Also known as "The Ravager," Chimera Prototype Mk-XXIII is an example of man's hubris. What was supposed to benefit the last bastions of wealth and power in a dying world has become an unstoppable force of destruction.
While technically alive, magnagogs have little in the way of personality and are driven via telepathic command.
The Qualm is a giant mass of tentacles and leaves with hundreds of mouth-like pods lined with teeth. It is about 1/8 of the size of Earth's moon, and it travels through space looking for nutrient rich planets that it can suck dry.
After the PCs defeat (or force the rapid retreat of) a villanous necromancer/demon summoner, they discover a book. This book outlines how to summon a demon whose power increases according to the size of the summoning circle used to summon it. And after the PCs examine a map of the country, they find that the layout of the cities and roads match up with the required summoning circle. In fact, the final road that would complete the circle is currently in construction.
With anyone who can find out about the circle and the ritual to summon the demon able to somehow make the final road/line, and destruction of the cities and roads currently in existence out of the question (unless this is an evil campaign- that removes the moral quandary), how are the PCs going to solve this problem?