Desert Province
The fifth province in Twilight Princess, it is accessible only by first going to Telma's Bar in Castle Town. After you have received her letter, check the map for Auru's location, and talk with him, answering yes to his question, and you will receive Auru's Memo. Give it to Fyer at the cannon in Lake Hylia so you can take the Oasis Flight to the Desert Province.
This province consists entirely of the Gerudo Desert. It is home to the Arbiter's Grounds and the Mirror Chamber, through which you can enter the Twilight Realm. The Cave of Ordeals is located here as well, at the Gerudo Mesa; it is accessible after you remove and replace the missing middle segment of the Bridge of Eldin, which blocks the entrance to the cave. One of the six Owl Statues is also found in this province.
This province, along with Eldin's Hyrule field is also the only area in Twilight Princess you can find warthogs in.