What map do I need to get there? How do I obtain the pieces?

You need the Uncharted Land Map Pieces. There's 9 of them:

[item=Uncharted Land Map Piece ][item=Uncharted Land Map Piece ][item=Uncharted Land Map Piece ][item=Uncharted Land Map Piece ][item=Uncharted Land Map Piece ][item=Uncharted Land Map Piece ][item=Uncharted Land Map Piece ][item=Uncharted Land Map Piece ][item=Uncharted Land Map Piece ]

You can buy them from the shop search or get them from random events. Once you have all the pieces, you take them here:

http://subeta.net/games/treasure.php?map=unknown

Thank you very much!