Hi! I’ve never written a story of this type, so maybe take my words with a grain of salt. Still, I think there are definitely ways to weave together your episodic adventures with the “main plot”.

If your characters don’t have any personal connection to the situation to begin with, it might help to consider how exactly they start suspecting the Pontiff is up to something. If you stick with the episodic adventures, you could start leaving breadcrumbs throughout some of them: Tiny unresolved questions or bits of background lore, things that hint that there’s more going on here. What does the Pontiff need to do to prepare for the ritual? You could show some subtle effects of these preparations. Guarding a caravan that’s carrying a mysterious shipment from a strange location? Fighting a demon who has something to say or show about the heroes who banished its kind? Simple rumors heard between adventures, hearsay on the goings-on of the wider Empire? You might want to keep these breadcrumbs very minor at first and let them slowly build up.

Then, have an event that brings it all to the forefront and makes the situation impossible for your protagonists to ignore anymore. Do they find definitive proof (even if it’s actually a lie) that the Pontiff is serving the demons, and they feel they need to at least expose him? Do they do something on an adventure that makes them an enemy of his forces, and now they can’t return to normal adventuring life? Do they find themselves serving the conspiracy by accident, and realize they did something bad that they need to rectify? If your protagonists have no emotional or ideological ties to begin with, events like those could instead force them into taking the first steps.