Embarked in a terrifying area, fortunately i did it while being prepared (increased embark points to get some extra items with the expedition)
So i managed to build my underground fort before the various undead managed to spot the entrance, and avoided this way the many many "foul smoke" (that can transform anything alive it submerge into a one of those nasty husk "enemy of life")
After a long while finally the first caravan of the nearby dwarven mountainhall appeared.
Their soldiers managed to make short work of some undead on their way to the depot and good news, no "foul smoke" anywhere.
The diplomat made some agreement with my leader, managed to trade several rock crafts against an iron anvil (to melt it into iron bars) , all was good, once everything was back in my stockpile, i rechanged my alert status to exclude again the surface small burrow for the civilians, so they would keep underground.
I waited for the merchants to leave, but when i went back to the surface to give a look, i noticed they were in a middle of a battle with several undead and monster husks. It went very bad for them, several dwarf corpses, "diplomat left unhappy" ... yeah sure.
As he left, i locked again the floor hatch over my entrance (fortunately the undead are way too dumb to figure out how to break or open such thing.
While i was expecting the very few dwarves survivors to run away, as a "foul smoke" was over them, they didn't and they faced even more undead and monsters husks, annihilating the caravan once and for all.
And now there are +/- 20 undead and monsters husk (some of the soldiers and merchant dwarves are now husk too) roaming around the trading depot, making it so i can't even get there to loot (there's a water buffalo husk that is a real dwarf killer).
I have no idea if the "diplomat" even managed to get away from this "depot of doom"
I'm not sure it's going to build a nice trading reputation to my fortress.