In order to progress through a certain point in the story of Final Fantasy 1 Pixel Remaster, you need to find the Rosetta Stone. You can check out this guide at Seeking Tech to see when it is located.
The Rosetta Stone can be found on the 5th floor of the Sunken Shrine. So, you need to head over to that part of the dungeon.
Once you get there, you will notice several small rooms. The stone is located on the top right most room. In order to get there, you need to move to the upper left opening.
Once you pass through it, you will end up being on the upper right opening of the map, which is where the top right most room can be accessed. Once you get there, you should find the Rosetta Stone in one of the three chests.
If you need a visual guide, then you can check out the video below: