Spoilers for Strange Aeons book 4, A Whisper Out of Time, follow.
So my party is wrapping up the Lodge currently (seems they may figure out the Stairs before they even start the vilkacis attacks) and I've got to start thinking ahead to Illmarsh.
While the latter third of Whisper isn't exactly good or thematic, I think that replacing the skum tunnels with the Mysterium (reflavoring it to be part of a weird collaboration between the Dagon cultists and some Abraxas and Shavaska cultists to smuggle demony things under Lake Encarthan; there'll be much more to the complex they won't have access to) will be fun.
Essentially, I want to drop the actual skum angle and make Illmarsh's dark secret much more up-and-front Dagon related. I know very well my players will *not* like the skum stuff, so I want to tweak it while trying to tie things together a bit more with the plot as I've developed it - the Way collaborating with other cults to secure an artifact but then aliens attack, as they do.
Has anyone here got any experience mixing AP books together? Are there any things I should avoid? I also plan to put the Colour in the "Mysterium" as well, because who doesn't want to fight that thing.