I was almost thinking of using body thieves who take bodies headed to Manifest and are selling them to beings who in turn use the corpses to create a magical castle that has a portal within it that allows a big bad guy to walk the Prime, without any binding. The grand villian being Orcus, of course. This premise has shades of the Glen Cook novel Shadows Linger of the Black Company series. The PCs might even sell cadavers to these creatures, not knowing the final resting place for the bodies. The 'castle seed' is a black rock that is deadly to the touch, but grows each time a body is fed into one. Lean times cause PCs to do strange things.
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