All of a sudden the ground is gone and they're somehow walking among the stars as they fall out of the sky around them, even though there's no ground.

The end of the universe should be the stars simply turning into black holes and maybe they are. Maybe, in a more grounded version of reality, the universe is simply cooling down, as entropy causes it to creep into heat death.

But this universe is ruled just as much by magic as it is by science, and so, even as the stars turn into black holes and dissipate, they also cascade around, falling from nowhere into nothing.

Also magical: the strange crystalline palace that they suddenly see in the distance. Reality is compressed here. All of time and space caved in at a single point. This palace is all that's left of the entire universe.