Bromine Lake is a lake that is almost a very small inland sea. It is also special for other reasons. It always has the warmth of a living Human, no matter what the season. It is not a hot spring with its sulfuric chemicals. There are hundreds of theories, but no answers.
Even if they did, they would never believe it, as it has ties to the time of legends, to the time of Corvus.
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?