To unlock this trophy you must first have a total of 80 Gold Bricks to access Huge Worlds, and Make sure you have a character that can Breath and Walk Under Water (Skeleton, Zombie, Mummy, or Deep Sea Diver).
While on the Galaxy Map press Square and select the option Locate a World, it is the picture of the world with a wrench on it.
Enter the following x12y34z56 and set the zone to J and find world.
Now x12y34z56 is not just the name of the World but the Location of the Legendary Brick in the World.
In this game X is East and West, Y is Height and Depth, and Z is North and South. Every map has a central Point being x : 0 y : 25 z : 0. Now you do not start at the center of the map but this is just an example.
So x12y34z56 would be North West