Three ideas for world ending finales to campaigns, but also a focus on how to foreshadow them in the earlier adventures.
What kind of portents would you use for:
A monster the size of mountain moving across a continent?
An industrial revolution by force?
A plague from the god of language that renders sufferers mute?
Weather changes. No, really, Read about rain shadow or Lee wave. I can make some sort of simulation for you (it would be one of my assignments with surprisingly little changes!) or try to explain it like that:
In case of Lee wave, you can suddenly make an area of fairly benign weather start experiencing severe thunderstorms (recall my post about supercells from a month or so ago :D) because what would be just a normal rain suddenly gets a lot of air with vastly different temperature, moisture and… updraft and downdraft. Post for reference. You have a walking atmosphere disruptor.
Other then that, if you have mining on any noticeable level, they could suddenly collapse from a sudden quake despite your creature being on the other side of the continental plate.
Read The Witcher books, in many respects Nilfgaard invasion is just that. Just to clarify, these are not some offshoots created after games but what the game was based on. Unless the games started their development in the '90s :P (a bit of a jab at this discussion).
I think that the best picture is in the The Tower of the Swallow. I can make you an amateur translation if you can't access it.
I think that this should be resolved by other gods getting a bit peeved that no-one can honour them with hymns and prayers ;).