From the Super Mario Wiki
The Dark Realm is the second of the two realms of Jewelry Land in Yoshi's Safari. It has more dark, mystical themed levels with more tough challenges. Once all of the levels are cleared, Bowser's Castle will appear. It contains only five levels. One consists of many huge platforms on a purple sea, a gigantic Ghost House with a background so huge it looks as if the mansion is the size of a city, a dark cave, Bowser's Castle, and a Floating Castle in the sky.