Death Sword is the mini-boss of the fourth dungeon in Twilight Princess, Arbiter's Grounds. Death Sword is an undead spirit, wielding a heavy, dark sword for a weapon. As Link first enters Death Sword's lair, he will come across the large, black sword embedding in the ground. Only when Link breaks the seal on the sword by slashing at the ropes with his own sword does the battle commence.
In the beginning, all Link will see is a giant sword flailing across the room. By turning into Wolf Link and applying his beast senses, the true form of Death Sword will be revealed. At this time Link, remaining in wolf form, will need to attack the spirit after it strikes with its sword. After a number of successful hits by Wolf Link, Death Sword will become fully visible.
During this part of the battle, Link will have to nail Death Sword with an arrow, the Clawshot, or the Gale Boomerang. A hit by any of these will cause Death Sword to rage across the perimeter of the room. Another hit will cause Death Sword to become vulnerable, at which point Link can attack it with his sword.
After Death Sword is defeated, a chest containing the Spinner will appear.
Death Sword Concept Art from Hyrule Historia