You could probably get all four novels and the short stories into one modern compilation, and I can't for the life of me understand why they haven't done so yet.
The entire environment of the Plain of Fear feels very much like a transplanted xeno-ecology, doesn't it? Question becomes whether:
A) It's an intrusion into the Black Company setting from Starsfishers, possibly the result of some serious magic (deliberate or otherwise) or a scientific catastrophe (maybe involving FTL/subspace experimentation).
or
B) Black Company is so far in Starfishers' future that the Plain is a last holdout of the native ecology in a world that was terraformed in some distant past by a scientific civilization that fell and was replaced with magic-using inheritors. That would have some similarities to the way Tekumel still has patches of the original (deadly poisonous) vegetation that spread from the "reservations" starfaring humanity (and allies) confined the native ssu, hluss, etc. to after their conquest.