We are just starting book 3, and one of my players has a Elder Mythos Cultist as his new PC. He's also very into the Corruptions from Horror Adventures idea, but isn't sure which one to use. My suggestion is the Aboleth Corruption, which I think will have fun tie in potential in Wake of the Watcher.
and he pursues the Whispering Way to recover the Seasage Effigy and put a stop to their "frozen world of undeath" agenda, which he sees as blasphemous to the ultimate entropy that Azathoth represents. He missed a couple weeks and explained his absence in game as having been called in for a meeting with Azathoth, who is unhappy with him.
This explanation doesn't really jive with Azathoth as oblivious and uncaring though, which planted the idea in my head that this wasn't actually Azathoth, but another creature which is taking advantage of his Unhinged Mind to plant secret agendas. Which ties in pretty nicely with the whole Aboleth Corruption. Still deciding which manifestations he should get...
So I'm thinking once he gets the party finds the Seasage Effigy and is concluding the adventure, this guy will have to make saves (maybe once an hour? It's kinda his big mission so I don't think one save should get him out of the woods) to avoid blacking out, stealing the effigy, and returning it to an Aboleth waiting in the water nearby.
I'm not really sure what to do with the Corruption before or after though. Anyone have ideas on nefarious things the Cultist could get up to? Stuff that could further an Aboleth's agenda and perhaps endanger innocents in the long term, but not be TOO disruptive of the story?