The King's Key is an item from Phantom Hourglass located on the Isle of Ruins. On the Isle of Ruins, Link will need to enter Doylan's Tomb and make his way all the way to Doylan's Burial Chamber. It is here that Link can speak with Doylan and he will give Link the King's Key.
Doylan will then tell Link to enter Bremeur's Tomb, back at the northwest corner of the Isle of Ruins. Once inside Bremeur's Tomb, Link can use the King's Key on the large lock, which breaks the great seal. This will cause water to subside across the entirety of the island, revealing new paths throughout.