I think the song is literally about the hopelessness of their journey, the certainty it will end in death for all. It is a despairing song. Beloved home is behind, the ever-darkening world is ahead - and there are many paths to tread - choices to make. (Recall Frodo telling Gandalf he wished the ring had never come to him. "So do all men who live in such times, but that is not for them to decide. What you nave to decide is what to do with the time you are given") . Pippin sees their road getting darker and darker until it is fully nighttime, at which point death will cause everything and everyone to fade. And it has a more universal application here, for all things pass thus. Yet the ending could be construed as positive, though not the way Pippin sings it. When you arrive at the very edge of darkness, the stars alight. And it is this light that chases away mist and shadow, cloud and shade. If the song is understood in this way, we can see Pippin desperately trying toimagine a positive outcome, despite his conviction at the moment that all will fade. It's very lovely, and so well-placed in the movie. Imagine making 3 flawless movies in a row.