I think the idea and the plot are great, the ending is cool but I would have loved a bit more in-world storytelling while we’re looking for the skull because it is such a cool story and I want to know more. A tome/letter next to each skull that tells more about the world we’re living in and maybe also explains why we’re doing what we’re doing would be really cool.
And I would absolutely lower the echo of the footsteps, they were distractingly loud even though I get the idea behind it (the echo signals loneliness and emptiness).