Build the Ship to access the Fragments of the Mysterious Diagram.
To unlock Mysterious Diagram you should find the first piece. The first piece is hidden on the second island of the Ship. Complete the Secret of the Gate quest and find fragments of the Mysterious Diagram.
The island of the Ship marked with the symbol of the diagram hides a piece of Mysterious Diagram.
When you unlock Mysterious Diagram you will destroy all the Evil Crystals made by Menes and free the City from the curse. Collect all the pieces of the Mysterious Diagram and build the City Gate.