The Golem Wars are over now. There is a great peace around the world, but something evil and powerfull has been awakened in the darkness.
A PC awakes in a cell plastered with blood. On the floor lies a crucifix necklace. She escapes, chased by the guards and soon the terrible truth of her situation dawns on her…
The heroes find a small crypt somewhere and being brave adventurers they want to check it for evil forces (=graverobbing). Or for any other reason.
In a village the characters hear rumors of a man that recently came through screaming of demon attacks and murderous insects.
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?