The Desert Island is where you end up if you try to navigate to Ninjinia or Kabukabu without the Navigation Book being fully deciphered. The terrain is desert, and there is one testing cave. The cave tests your knowledge of kanji compounds.(Navigating to or from the island you will be attacked by sea slimes that teach kanji compounds)
You will need to know 100 words in order to beat the cave, and unlock your first spell. Once you have done this, a second deserted island will appear in this ones place.