From the Super Mario Wiki
Special 1 is the first extra world of Super Mario 3D Land that the player unlocks after defeating Bowser in World 8-Bowser's Castle 2. It takes place in a sea green field at night with flowers and hills in the background. The map music is a remix of the game's World 1 map music. If Mario defeats Dry Bowser in the last stage of this world, Luigi is saved and becomes a playable character.