Now I noticed a very easy to justify difference between the Drezen in the Worldwound Campaign Setting book and the Drezen described in the Pathfinder Adventure Path. The setting book states that Drezen has a population of 7,489 humanoids with roughly half as cultists and the other as prisoners and slaves. The AP only has a several hundred tieflings and cultists - likely because the majority were slaughtered or marched down toward Nerosyan with the bulk of the demonic forces.
Still it does provide some food for thought for those hoping to expand the game even further. Firstly think of their fates. Were they butchered on the streets as a bit of fun before the demons left en masse? Plenty of excuses for subsequent hauntings and occasional more powerful undead. Marched out as meat shields? Reports from afar describe them as such. Pinned to the walls half-alive for the PCs to locate? Driven to hide in the nearby Vescavor caverns or other more hidden cave structures? Entombed in basements ready for the builders to accidentally unleash? Infected with demon plague for similar purposes?
Hell I'm pretty sure I read somewhere that the initial inhabitants in Drezen were wiped out by abyssal contamination that drove them mad and made them kill each other. If you like, you could have a bunch of Warped Ones still sealed up in an old ruin who could break out and cause some action at home if you like.
I'll have a bit of a think about how I'll introduce this little fact at this late(r) stage of the game and will put up a post with a few other mini-quests.