You learn from Zelda's diary that a new Champion's Tunic was made for you and is now hidden.

Quest Location

You can start the quest by reading the diary in Zelda's Secret Well.

How to Complete the Quest

You can reach Hyrule Castle by launching yourself with the Lookout Landing Skyview Tower, then glide north. You may need extra stamina to get there. Once there, enter the Sanctum in the center of the castle. The throne overlooks the center of the Sanctum. Go up the stairs to the throne, then use Fire Fruit, or flint, or a fire weapon, to light the two torches near the throne. This will open the stone behind the throne. Open the chest inside the stone to get the Champion's Leathers. This completes the quest.