Inquisitor Zacris (as all powerful controlling creatures seeking to snuff out opposition need) had a network of spy-enforcer agents at his disposal.
The most powerful and loyal of these he imbued with super-human demonic power. (I'm thinking some kind of re-skin of a half-fiend or a half-dragon, aberrations if you prefer...with wings, of course, to facilitate covering large amounts of territory/distance. Maybe 6 or 10 Enforcers total. Or more if you like. Different tiers of creatures with different special powers, perhaps?)
These mutated monstrosities were imbued with invisibility, anti-detection enchantments, resistance to -well- practically all magics/energies, inhuman strength and endurance, and physical invulnerability (like, can't be hit by anything less than a +3 enchanted weapon or the 4e equivalent. Anti-demon weapons/spells do
partial damage?...something like that.).
They were responsible for carrying out many missions for the Inquisitor, ravaged many a land, razed many a village and town and were responsible for the particularly brutal captures and murders of uncounted "enemies of the Empire" (at Zacris' command, of course. Usually they were for information gathering and retrieval..so Zacris could have the pleasure of judging and passing sentence himself).
The only thing Zacris could not enhance them against was the particularly rare Snow Daisy...sacred to <insert "goddess of purity, peace, love, what have you" here>. Weapons dipped or coated in an infusion derived from the flower's particularly fine petals and pollen could pierce the creatures' hides and diminish their supernatural powers.
The enforcers would flee from a Snow Daisy's very presence. (I'm thinking like cats were in/to The Mummy
A children's rhyme/game survives to the present day:
"Ring the daisies round and round.
The demon-men are coming down.
Ring around the petals snow.
The demon-men are sure to go."
(the tradition of women wearing wreathes of white daisies at festivals and weddings can also be attributed to the legendary Snow Daisy's protective properties.
Knowing of this apparent weakness (and infuriated by the personal failure) Zacris was ever watchful for the plant and sought out the aid of the Phoenix Brigade (multiple times) to torch areas where the flower was found or even
might be able to grow.
This led the once lush (and populated) slopes and valleys of the <insert "spooky mountain range" here> to become the blackened, broken and dead terrain it is today.
As tends to happen in such things, some druids (or druid, singular?) have managed to keep very few of the already rare plant alive and hidden. (Something the party would have to learn in the first place before even attempting to find one. Well, really...
1- Learn about what the enforcers actually are.
2-Learn about their vulnerability to the snow daisy. Thought be extinct. 3-Learn that they
miiiiight still be found in X.
4-Find X. Get the daisy(-ies).
5-Make the potion.
9-Find enforcers. Apply potion to weapons. Kick butt.)
With Zacris' passing from the realms, the enforcers entered into a magical stasis in the lower tombs of Zacris' walking tower and passed into horrifying legend. The return of Zacris' consciousness/soul to the mortal world has caused them to arise, but his lack of memory/power has them scattered and confused.
Rumors have recently begun to circulate of demon-men scouring the skies above the lands of the ancient empire (perhaps over lands that are no longer part of the empire but once were?). News of strange atrocities encountered with no apparent cause have made their way into the cities.
The entire town of <insert known but not particularly easy to get to town" here> was found to be coated in the blood of its populace. The entirety of its inhabitants, from the eldest to youngest, bodily torn apart and splayed throughout the streets, homes and shops. The terrified merchant whose caravan discovered the town supposed an invasion of trolls...but then, why were the bodies not consumed?
Is it possible the Inquisitor's Enforcers have returned to search for their dark master? And without his power and will to direct them...what might their demonically twisted (yet limited) minds be capable of?
These guys would be...well...pretttty much impossible to defeat and (as I'm picturing it) should be encountered one at a time (at least til the party could find a snow daisy and/or became very high level).
Good luck and happy gardening.

--Steel Dragon