Raftel

Raftel is the island at the end of the Grand Line which supposedly holds the treasure One Piece. There are four poneglyphs known as Road Poneglyphs that, when read together, lead to four islands which, if aligned, will lead to Raftel's location. One of these is located on Zou, one is at an unknown location, and the last two are each owned by the Yonko, Kaido and Charlotte Linlin. Little has been seen of the island, but its shore appears to be composed of steep cliffs with large amounts of wilderness, though there are some objects that appear to be shaped like buildings or fortifications.

It is said that upon reaching Raftel, if one has taken with them along their journey the passages of the poneglyphs, then the Rio Poneglyph will finally be revealed. Furthermore, the Road Poneglyphs, a set of four special red poneglyphs, each reveal a point on a map of the world once read. When all four points are deciphered, it becomes possible to discern the place at the center of these points where they all intersect, and the true path to Raftel will be made known. Gol D. Roger and his crew are the only known people to have reached the island. Roger offered Whitebeard the information on how to get to Raftel, but Whitebeard declined because of his own dreams of achieving a "family" of crew members instead of reaching One Piece.  

Trivia - From what has been shown regarding the Road Poneglyph points, the four marks form an "X" when they intersect, indicating where Raftel and the treasure One Piece await. On a traditional treasure map, the location of the treasure is generally marked with an "X". The image given make it looks like "An X made by Xs", hinting at just how important of a location it is.