It's effects are unknown.
To us, maybe. But if this herb was some useless nonsense plant, it would not have earned Mina any opposition. People knew it did something back then; if we can get our hands on a herbalist's catalogue, we'll see the herb somewhere. We may need to get an old one, though.
Edit:
Oh, and we should look into the gray king's insane dwarftasks and projects. Perhaps he was studying dwarf psychology, or attempting to build a superweapon. It would explain why he went murderous; the people he killed came too close to his research.(The fool should've gotten a dumber wife with a poorer memory. Why'd you think I prefer the senile?)
Edit2:
A book about herbalism will be multipurpose, and especially usefull if we decide to venture out of the fort.
Edit3:
Maybe we could disrupt the aquatic ecosystem near human towns by releasing vicious, aquatic cavecreatures (War-trained by a elven vampire, perhaps?) in their rivers. They'll eat the fish dry, robbing the humans of a food soure.
We could also try to dump large quantities of toxic metal(Lead, tin, maybe zinc if we can find enough?) Just put all of it in a large, heavy cage(made of lead, maybe) dump it into a river and wait for the humans to die. Maybe we could also utilise the large quantities of female animals we'll have, and dump loads of sour milk into the river.
Edit4:
Killing the humans will leave the field open for the goblins. Their rapid expansion suggests a demon at work; it'll take but one well placed catapult shot to take the scrawnier ones out. If we're up against a tougher demon, I would guess we'd just draw his attention with a catapult, and have to resort to the gauntlet to kill him. I guess we'll have to get creative to capture whoever uses it.
The user of the gauntlet will have to be someone of great willpower and fighting skill. We'll point another catapult at his location, and have a bunch of seasoned warriors with nets (I'm guessing these will be made from heavy chains or something) stationed nearby. We'll make the warrior kill the demon, and sound one horn to call the warriors in to capture him. We'll let the warriors restrain him after throwing their nets(if he manages to resist the gauntlet, he'll coöperate.) and let a medic inject him a tranquilizer. Then, we'll remove the gauntlet(with disenemberment if we have to) and tend to his wounds.
If all fails, we'll sound the second horn, signaling the catapult to launch at the gauntlet's location and the warriors to retreat.
Without leadership of demonic strength, the goblins will feud instead of being a real threat. We can then either play them out against one another, or side with their most altruistic faction.