The plot was written very nicely, only few things I was able to pick out was:
"The only limiting factor is physical limitations such as areas accessible only by small passages, requiring the player to gain the fish form before entering."
This sentence gives sort of spoilers and I think it would sound better if you stayed a bit mysterious by saying something like "getting past a certain obstacle with a certain form you need to find."
Second, the kid/creator only remember the verse of the lullaby.
Finally, if I remember correctly, after the creator has been destroyed a lot of the civilisations came back, but I'd have to finish the game again to see it once more and be sure.
Oh and there's one small typo in the last sentence.
