Just found these. I like, but there are a few problems with the "Custom" insert that really drive my inner Grammar Nazi up a wall.
"For a short time, the peace and prosperity that had existed for 800 years before was returned to the land."
Remove the two words I've made bold. The "for" implies that Ys had been peaceful for the 800 years leading up to the present day, which is definitely not the case. The "was" creates some really awkward phrasing.
"But it is up to you, great adventurer, that must guide him!"
It seems like you changed this sentence halfway through writing it. I would suggest either this:
"But it is up to you, great adventurer, to guide him!"
Or this:
"But it is you, great adventurer, that must guide him!"
Also, while I'm at it, might I suggest changing the word "great" to "brave"? It seems a better fit, not just because it works better with the adventurer idiom, but also because it seems a bit presumptuous to call the player "great" when he hasn't proven himself by beating the game yet.