You know, it occurs to me that the ol' goat isn't very smart. He wants your kingdom full of adventurers to leave. Why? Presumably, because you're invading his territory and killing his minions. How does he want to drive you out? By crashing your economy by flooding your market with monster trophies. These trophies are monster body parts; most of them vital to survival. Sure, maybe goats can spare their horns, and serpents have a spare fang or three. I'm not sure what a changeling's essence is, but it sounds pretty vital. A de-animated helm requires the destruction of the creature by nature.
So all in all, the big plan is to sacrifice his minions to cause you an economic boo-boo in hopes you'll stop killing his minions. Basically, he's beating you to the punch I guess in hopes that your people will riot and it's lulzier to watch them tear you apart themselves, assuming a crippled economy is enough to do that?
I dunno. Doesn't seem like the best strategy. Granted, there are some real world countries' politics which have at times adapted similar strategies (whether or not that worked especially well for them...) so maybe I should shut up.
(Yes, I am entirely aware I am grossly over-thinking this. Although it's nice to have a villain who not the machine-made "muhahaha I will kill you all and steal your souls" I just wish his creative plan made just a tad bit more sense. Not too many villains strike your economy point for massive damage, so it's cool if nothing else.)