Need some advice on campaign pacing as a DM
>party is changeling druid et al
>first adventure was defeating a cult of jubilex
>they were supposed to find a letter linking to a larger cult hideout in far off mountains but I forgot
>main plot involves mysterious incursions from the shadowfell
>changeling druid is having dreams about them
>they're going to find a staff for a wizard on an unrelated quest now
>about to explore a druids tomb
>ambushed by 3 shadar-kai who will give them an offer they cannot refuse
I don't know how to work the cult of Jubilex back in now.
I doubt they'd work with a shadowfell entity.
It doesn't make sense for them to just coincidentally be at this druids tomb, 100+ miles from the town they were first encountered in.
I guess when they finally return to the starting town, I could have another issue with the cult, but that might just make the characters paranoid that every time they leave their starting town, something will go wrong. I had that in a campaign once, and it worked well for it, but it wouldn't be a good trope for this one.
I could try to reintroduce the letter but they dealt with the cult hideout pretty definitively. I guess I could say that the guard captain and his men went on patrol there and found a letter in an area they hadn't checked, but it's just gonna feel ham-fisted especially given how through the PCs wee in searching the place.

Any advice? The games been flowing really smoothly so far so I want to reintroduce this cult in a way that makes sense. How would YOU do it if you were DM?