So I had this idea today. The scenario strikes out thusly...
Cultists of Rovagug come to a startling realization. They cannot release their lord, but should he perish his soul would escape its cage and journey to the Boneyard. Such an immensely, omnipotently powerful soul would likely retain its power.
Through eldritch machinations, they complete this goal. Rovagug's dying roar echoes throughout Golarion and beyond. Many are immediately struck deaf and even blind. His writhing body flails against his cage as his essence leaves it. The earthquakes caused by this ravage Golarion. Large swaths of land collapse into the Darklands beneath.
Abruptly, contingency plans by Sarenrae and Asmodeus come into effect. All passages from the Astral Plane out into the Outer Planes slam shut. This is no longer the Prime material plane, but an offshoot; it is an errant thread that has been snipped from the tapestry of reality that is the multiverse.
All Outsiders affiliated with any Outer Plane immediately and painfully perish - even those merely summoned to existence. Planetouched beings with Outer Plane associations are nauseated; those who recover from this are permanently sickened.
Divine casters are stripped of their powers. Arcane magic begins to defile the land, as plants turn to ash and the earth becomes barren. Any spells that would touch upon an Outer Plane fail. Alignment rapidly becomes subjective without the objective truths in place.
Kyonin is utterly destroyed, sinking into a massive pit of black blood. In time, the foul liquid enters the rivers on the surface, and spreads its corruption around the world.
In the Astral plane, Rovagug's ancient and murderously angry spirit devours the wayward souls whose path has been so abruptly destroyed.
Razmir becomes the One True God, saving his chosen followers from the Destroyer by capturing their souls within family heirlooms. Meanwhile, pockets of devout followers of the lost divinities hide, trying desperately to see the chaos as simply a test of their faith.
Anyway, this was just a little idea I had earlier and thought it might be interesting enough to share.
Cool, but... what's the adventure, other than mere survival?
That's a pretty interesting thought. But as VRMH said, did you have some adventure plots in mind? Survival is a big thing, but there's plenty of other stuff you could run with this. As Big G**@%$n Heroes, perhaps things like trying to evacuate a city falling into the darklands, by moving the people into a somewhat habitable part of some inner plane (likely the elemental plane of air or earth)?