Iron Knuckle

The Iron Knuckle (アイアンナック, Aiannakku, "Iron Nuck") is one of the toughest sub-bosses Link will have to defeat. It swings a mighty axe that deals devastating blows to whatever comes in its way. If Link is hit by this axe, he will lose quite a large amount of life energy. In Ocarina of Time, Iron Knuckles are associated with the Gerudo. Though quite similar to Darknuts, Iron Knuckles are empty shells powered by evil, whereas Darknuts have a physical body.

The Adventure of Link
Iron Knuckles can be found in many dungeons and caves in The Adventure of Link. They constantly defend with a shield that they move up or down to block Link's attacks and have a high amount of health, making them one of the game's most difficult enemies to defeat. A good strategy for defeating these enemies is to jump, and crouch stab at its head as you fall, thus hitting both its top area and its lower area, guaranteeing damage.

Rebonakku
This Iron Knuckle (known as レボナック, Rebonakku in the Japanese) rides a ghostly horse, and is the boss of the Island Palace. Link must use the Down Thrust initially to damage him. After enough damage is done, he dismounts his horse, and can then be fought as a normal Iron Knuckle. The trick is to attack while standing and ducking down so that he can't block every attack with his shield.

Ocarina of Time
In Ocarina of Time, Nabooru is captured and brainwashed by Koume and Kotake, a pair of Gerudo witch sisters. Koume and Kotake then force the brainwashed Nabooru to don armor and protect the entrance to the final chamber in the Spirit Temple as an Iron Knuckle. When Nabooru is defeated in her Iron Knuckle form, the armor falls away and reveals her, apparently releasing her of her hypnosis. Immediately, the witch sisters appear and threaten to brainwash her again (suggesting that the hypnosis was temporary and had to be maintained).

After Link defeats Koume and Kotake, Nabooru becomes the Sage of Spirit and is freed from her hypnosis permanently. The fact that the other two Iron Knuckles in the Spirit Temple dissolve completely when they are defeated suggests that they were animated by magic and not hypnotized people, like Nabooru. In both Ocarina of Time and Majora's Mask, Iron Knuckles come in three colors: silver, white, and black. Also in Ocarina of Time is the armor that Nabooru is encased in, which looks slightly different to others.

Iron Knuckles have two "stages" — with armor and without armor. While wearing armor, they're slow and impervious to anything save sword swings and bombs. Once enough armor is removed from attacking them, they become much quicker, but with the added effect of now being weak to arrows, making a long-range attack strategy is effective. However, in the Child half of the temple, Link may enter and leave as he pleases during the battle, as the door is not restrained.

Majora's Mask
There are three Iron Knuckles in Majora's Mask and they fight just as they do in Ocarina of Time. Two are beneath the Ikana Graveyard on the second and third day respectively, one guarding the resting place of Flat, and the other guarding a Bottle. The third is on the Moon in the hide and seek game played by Link while in his Hylian form.

There is also Gold Iron Knuckle armor on display in the Curiosity Shop in Majora's Mask.

Trivia

 * Onox, the main antagonist of Oracle of Seasons, highly resembles the appearance of an Iron Knuckle. The only real difference is that Onox wields a flail while Iron Knuckles tend to use axes.