All you need to stop a brain is a bullet.
"Alright where is the beast so that I can size it up?"
"That's the thing...you see that mountain range over there?"
"Yeah, the twin spires, what of it?"
"Well that's the beast"
"Bah, dere be no such thing as no 'peripheral beast'. How can their exist a creature you canno' even look at?"
Torax Shieldbreaker, Dwarven explorer
Rabbit-leonine like grass dwellers, for flavour
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?