The Myublin rises somewhere in the Moon Wilds, runs east out of them, and joins the river system that drains toward the Gorathi coast. On the map it is one more jungle river. In practice it is a line, the one the empire decided it could hold after it admitted it could not hold the Wilds. East of the Myublin, Gorath keeps patrols, watchtowers, and a string of small river-forts. West of it, the empire keeps prayers.
It is not the line because it cannot be crossed. The Vexlings cross it whenever they choose, and so do the runaways the empire would rather they did not. It is the line because it is the last place Gorath can mass soldiers on open, cleared ground. Fighting Vexlings inside the Wilds is suicide; fighting them at the river, with the trees cut back and a wall at your spine, is merely dangerous. The forts exist to turn the worst kind of fight into a survivable one, and to give the marches enough warning when a brood comes east.
The river itself does not make travel easy. Rapids, submerged snags, and channels that shift with the floods make the Myublin treacherous to anyone going upstream, and going upstream means going deeper into Vexling country. Few Gorathi do it willingly. Those who must, the scouts and the supply boats for the farthest outposts, move by night and keep quiet, betting that darkness and silence will let them slip past the eyes on the banks. The bet does not always pay.
Cross the Myublin and you have left the Empire. The Empire will not tell you so, because the maps say otherwise. The water knows. — a Myublin garrison saying