In a world where teleportation magic doesn’t exist, the opportunity to travel a few days journey in moments is beyond value. With only a simple toll, you can be practically on the doorstep of Nizul-tibi before you know it.
A plague has hit the local area.
In humans it affects only the most vulnerable, the children and the very old, and even than it's little more than a summer cold. 24 hours of sniffles and then it's gone, barely noticeable really.
To sheep however it is fatal and the whole economy of the area is in serious trouble. If this keeps up the area could well be facing famine.
Somebody (enter the PCs) must find the rare herbs needed to make a cure.