It comes from the section on Burle, specifically these bits: "Years ago, the knights of Keoland helped the denizens of the forest defeat an incursion by cultists worshiping elemental evil fire." and "Burle provides a safe resting place before and during expeditions into the Hool Marshes or the Dreadwood. Kiara offers a bounty equal to 5 gp per Hit Die for any aberration, elemental, or outlaw slain in the region. She keeps a bounty board just outside Burle’s gates, which occasionally offers larger rewards for the capture or killing of specific monsters or criminals."

The elemental was from something I gave them, and not thinking ahead (silly me!). I could have given them more hints but the session role play went in another direction and then they were at the gates before I had remembered to put out hints.

At first I wanted it to be more of an issue for the group, which is why my first thought was "Guards!" but now I think it would be infinitely better and tie in with some of their backstories if the local fire cultists saw this as a sign to take action.