Death's Door

This can be unlocked as soon as you get the hookshot spell if you so choose by heading back to The Stranded Sailor and, taking the path back towards the cemetery use your new hookshot to cross the water. You will have to defeat a mini-boss through the upgrade door, which will give you the upgraded hookshot spell that allows you to swing your sword while pulling yourself towards an enemy - smashing! Reference this video at 0:02 to find the location: