Shadow Nightmare

Nightmare is the final boss of Link's Awakening. It takes on six forms, or Shadows, of former bosses Link has defeated in Link's Awakening, as well as that of A Link to the Past. Nightmare is the primary antagonist in the Wind Fish's dream on Koholint Island, and is the force single-handedly responsible for the forced, continued slumber of the Wind Fish. Interestingly, Nightmare is self-aware: at the beginning and end of the battle, it reveals its plans to continue the Wind Fish's slumber, and speaks of how its death will signal the end of "its world", respectively.

Shadow Bot
The first form is a Giant Bot, which is, in essence, a dark blob. The Giant Bot is the easiest and least demanding form of the set; if Link simply sprinkles Magic Powder on it three times, it will morph into its next form, Agahnim.

Agahnim's Shadow
The second form is Agahnim, the evil sorcerer from A Link to the Past. This form can be defeated in the same manner as the true Agahnim was in his in-game appearance during A Link to the Past, by playing a grueling game of Dead Man's Volley. Link must parry balls of magic back and forth against this form, until Agahnim tires of the volley and submits to backfire. Similar to the true Agahnim battle in A Link to the Past, some blasts of magic cannot be reflected. After this form is dispelled, it shall transform to that of a Moldorm-like shape.

Oddly enough, if timed properly, Link can also use the Shovel to deflect Agahnim's blasts, presuming Link has not traded it for the boomerang, in a similar manner to how Link could use the Bug Catching Net to do so in A Link to the Past.

Moldorm's Shadow
The third form will be that of the first boss in Link's Awakening, Moldorm, in technique and in attack strategy. Striking the tail continuously as it rampages about the room, similar to its original incarnation, will accelerate the demise of this form. However, this form may slow down during the battle, and when this happens, Link's entire attack strategy must be repeated in order to deefeat this section of Nightmare. There is also another, more simple way to defeat the Moldorm; as Nightmare is switching forms, from Agahnim to Moldorm, Link may quickly and continuously slash the transforming mold, slowing and eventually halting the process. Only the head of the Moldorm will successfully form, and after more striking, the boss will simply change into its next shape, or at least sustain enough damage that Link will be able to defeat it quickly and painlessly as compared to its full-bodied form.

Shadow of Ganon
The fourth form is a Nightmare version of The Legend of Zelda's archetypal villain, Ganon, taking on his appearance from A Link to the Past. The Shadow of Ganon will summon a flurry of Blazing Bats in Link's direction before attacking with its Trident and moving across the room, much as Ganon did in A Link to the Past. To defeat it, Link must use the Spin Attack, or alternatively by using a combination of the Pegasus Boots and his blade, Link can charge at the shadow in a Dash Attack in order to defeat it. After about twelve hits with Link's normal sword it is defeated, and it shall morph into the Lanmola form.

Lanmola's Shadow
The fifth form exists as an A Link to the Past Lanmola, but is defeated in a different fashion than its familiar counterpart. The Spin Attack, Hookshot, Boomerang, Bomb, or Magic Rod will defeat this form. In addition, Link can dash into it with the Pegasus Boots. Upon vanquishment, it will morph into the final form, Dethl.

DethI
The final form is the only new enemy encountered in this battle. DethI exists as a central eye, from which two dark, muscular arms extend outwards. Using the Roc's Feather, Link must jump over the arms to avoid injury once they begin to swing, and at this time, the center eye will be hidden. Occasionally, the eye will open during the battle, upon which Link must strike it with an arrow or the boomerang, each time. Sixteen direct shots in the eye with the bow and arrow is enough to defeat it, though a single direct hit with the boomerang in the center of the eye will defeat it instantly. This form will take the longest, but once it is defeated, the Wind Fish will awaken.

As well as being a possible inspiration for Vaati, due to the similar central eye and overall form, DethI also appears incredibly similar to certain characters in the Kirby series, resembling the Kirby games' antagonist "Dark Matter", as well as the final boss of Kirby 64: The Crystal Shards, "Zero Two".

Aftermath
Once the Nightmare is dealt with, the Wind Fish will awaken. He will confirm that Koholint Island is a dream world, and that Link was absorbed by his dream the moment his ship was struck by lightning while he was sailing. By using the Eight Instruments of the Sirens, Link gathered on the island to play the Ballad of the Wind Fish, the Wind Fish finally awakens, dispelling his dream world and returning Link to the sea, from whence he came. He awakens on a piece of his former raft and watches as the Wind Fish flies overhead. The dream island of Koholint would only remain in his memories.

Trivia

 * Nightmare's final form resembles Vaati's final form in The Minish Cap and Four Swords.
 * The wanted poster in Ocarina of Time's Shooting Gallery bears some resemblance to Nightmare's final form, though there is not a proven connection.