I am currently running a campaign that I have set in a world that is fading slowly. The idea is that, while the players cannot stop the end of the world (it's kinda similar to the heat death of the universe), they will eventually be able to influence exactly how it ends, and what comes after. In a way, the big thing I am building towards is not stopping the end of the world, but instead changing the results of this ending. And, there are a lot of NPCs who will be very interested in having a different outcome than what the players would like.
Since the campaign takes place over multiple months + years ingame, my current plan is to slowly start introducing symptoms of things fading, like stars disappearing from the sky, plants losing their color, and rare animals just... disappearing. Eventually the symptoms could get more severe as the world approaches its end.
You must understand that big thing I want to do here, however, is have the world end with a whimper and not a bang. I want it to be gradual, but still present. With this in mind, something that is worrying me about my plan is that I am not entirely sure how to deal with this: If the world is ending, how do I deal with the people in it? Do they just die?
If I start killing off massive amounts of people to bring about this ending, suddenly the campaign takes on an extremely dark and morbid tone, which is not really what I want. I would be fine with a somewhat "sad" or "serious" tone some of the time, since obviously the loss of so many people isn't exactly a good thing. However, I am not sure exactly how I set up the world so that this does not accidentally turn into the most depressing campaign I have ever run.
Do any of you happen to have any ideas about how I can accomplish the disappearances of large amounts of people without ruining the tone of the campaign? I've thought a lot about this and still am struggling to come up with a good plan.
For reference, the game is Swashbucklers of the 7 Skies; However, this isn't a system-specific question so feel free to ignore that part.