I've picked up WFRP recently, and to get a better handle on the fluff, I've been devouring novels and crawling through forums and fansites. I think I've got a handle on the setting, but I'm planning a new adventure arc and I'm trying to decide who the antagonists will be.

Basically, the characters are sent far beneath the city, in the sewers of (Nuln/Altdorf/other major Empire city), to hunt for the source of the recent surge of monsters and mutants emerging from the sewer. In the depths, they find that a recent earthquake has opened up and revealed a large node of warpstone in the rock htat is corrupting the surrounding area.

Now I know that warpstone has many uses for skaven and certain unscrupulous alchemists, so that's two factions out for the stuff, but I want to throw a third party into the mix -- how do Chaos cults use warpstone? Tzeench cults come to mind, seeing how they savor mutation, but do the other cults (slaanesh, nurgle, khorne) use warpstone extensively? Are there any other bad guys i can throw in there? I'm thinking it'll be like a gold rush for evil people, and the players will be caught right in between.

Any help or links to pertinent info would be great.

The first thing that comes to mind is Skaven.

But as you say, Tzeenech, cultists in general, dark mages, witches, mages that just want that bit of extra power boost, alchemists (I had a apotechary/doctor that mixed small amounts of warpstone powder into his concoctions to slowly spread mutations through his medicine).

Followers of the Yellow Fang might come in handy. Granted they are really just cult worshiping minions manipulated by the Skaven, but it lends some breadth to the types of enemies the party will face beyond "mutant" "mutant" "panda...I mean rat ogre" and would let you unveil the Skaven later on to keep them mysterious and maintain their status as worst kept secrets in the empire .

In terms of sources...come on brain...one of the Mathias Thulman witch hunter books has a small town devoted to the Yellow Fang Cult (or at least a cult that could qualify). And they are mentioned in the section on lesser and forbidden cults in the 2nd edition supplement Sigmar's Heirs. I haven't read any of Gav Thorpe's skaven-y type stuff in a long time so I can't recall if they deal with the human servants of the Skaven at all.

AND..evil alchemists using warpstone in their concoctions is a good standby as k7e9 mentioned

I would say that there are probably College wizards who would genuinely wish to enhance their magic for the good of the Empire who could easily misguidedly decide that warpstone could be employed for good. Or even if they acknowledge it is utterly dangerous now, if they experiment on enough of the stuff, they might discover a safe way for it to be harnessed. This idea might even have secret support among some of the higer ranking wizards of a College.

One of the good old classics: Undead.

Liches, Lichemasters, Necromancers.
One of the more fierce wars the Skaven ever fought, was against Nagash, and over Warpstone.

If I remember correctly, Warpstone is really solidified Dhar magic, meaning it's the equivalent of a little Duracell battery for any powerful undead.

Ah, undead. Necromancers would certainly be a welcome addition to the mix. Nisses, where can I read up on the skaven-Nagash war?

http://whfb.lexicanum.com/wiki/Nagash

Not as detailed as you might like, but it's a start.

The basic story is:

- Nagash found this mountain, Cripple Peak, that got its name by being shattered when a massive Warpstone meteor struck.
- He started mining the warpstone & would create Nagashizzar on this site.
- Skaven at the same time (or later?) found the warpstone through their underrground tunnels and started mining it

- They meet at some point, and battle ensues.

- Skaven have a near infinite amount of warriors they can field, and so can Nagash. So after several generations of battle, Nagash proposes a truce.

- Skaven agree, but after a while decide they would be better of with Nagash dead, and create the Fellblade, a 2H sword made of Warpstone.

- They deliver it to a human prince (of Khemri?) in the dungeons of Nagashizzar, and when he grabs it, the 12 Skaven Lords mind-control him through it into hacking Nagash apart.

- Clan Rikek claims Cripple Peak for the Skaven.

- centuries later, Nagash returns, obliterates the clan Rikek that lives there, but finds most warpstone mined by that time.

The 7th edition army book of the Skaven have this story in much more detail.