New Adventure Path idea: For Profit and Pain
The all evil group is composed of young Kuthites novices seeking out several unholy artifacts for their cult and its master, a unique Kyton bound in the material plane. The group could start off in the material plane, eventually ending up in the cities on the plane of shadow.
At least one of these artifacts of suffering is used to create Liquid Pain, perhaps the most sought after resource in the lower planes of existence. The rest have a far more sinister use, which may somehow fit into the Kyton's plans of a grand escape.
Sounds like an interesting AP, though I might be careful about making the party all play Kuthites. Should be easy enough to have the kyton employer and then half the party is likely to be Kuthites by player choice.
Is this the same Liquid Pain as 3.5's Book of Vile Darkness had?
Might also be interesting to have the kyton not as tied to ZK as usual. After all, the kytons are attempting to force evolution, to graft themselves into perfection even if it hurts. They want to help you too! ZK is just pain with no other goal. Very angsty teenager attitude, with no clue about the S&M lifestyle he picked up other than "It looks scary and makes me miserable, so no one else could enjoy it either, right?"
Might also be a chance to bring back everyone's fsvorite elf from CotCT, Laori Vaus! Whether unredeemed and working for ZK again, or perhaps having broken from the church and spreading the word about SSC.