The jackalwere is none too brave, and confronted with angry warriors and greatly outnumbered, it isn't long before he goes to the bargaining phase of the encounter.
"Let me goes, and I'll tell you all of the secrets!" A few more cuffs and arm wrenchings and he spills.
The Jackalweres are from (wait for it...) the Untamed Hills. There, they were hired by a heavily armed orc with the unlikely name of Ubar-Sin. This Ubar-Sin is a very dangerous fellow, and an alchemist to boot. He concocts these noxious poisons and has his jackalwere hirelings deliver them to the area. They have been using the old forest fort as a base of operations, but they're not quite ready for the main invasion. Apparently, the JWs have been told, when the goody-goodies have been softened up by poison, plague, and other demonic surprises, then the warriors of the Untamed Hills will swoop down and use the fort as a base from which to loot and plunder the Duchy of Harcourt.
The small band you encountered were to return to Ubar-Sin and obtain more barrels of poison, which they would take as far as the river upstream of Marian Town.
Lastly, he is able to tell you that Ubar-Sin has his laboratory in a cave at the far end of this valley, and it is guarded by less than a dozen jackalweres, several of whom are likely to be running supply-getting errands for him.