Shadow Nightmare

The final boss of Link's Awakening, takes on six forms of former bosses you have defeated in Link's Awakening, as well as that of A Link to the Past.

First Form: Giant Bot
The first form is a Giant Bot. Simply sprinkle Magic Powder on it three times, and it will switch forms.

Second Form: Agahnim
The second form is the evil priest (or wizard in the U.S.) from A Link to the Past. Agahnim will take the second form and if you faced him in A Link to the Past, you know exactly how to defeat him. Send his magic ball back at him with your sword and it will switch forms once again. Watch out though, some blasts of magic cannot be reflected.

Oddly enough, if timed properly, you can also use the shovel to deflect Agahnim's blasts (presuming you haven't traded it for the boomerang).

Third Form: Moldorm
The third form will be that of the first boss, Moldorm. Striking the tail will cause this form to disappear. However, if the Moldorm of the Nightmare form slows down during the battle, the process must be repeated. Once Moldorm is defeated, it will switch forms again.

Fourth Form: Dark King Ganon
The fourth form is Link's worst enemy, Ganon. Ganon will attack with his ax, as well as sending demonic bats to attack Link. Using the Pegasus Boots and the Sword, Link must charge at the Ganon form in order to defeat it. After it is defeated, it will switch forms again.

Fifth Form: Lanmola
The fifth form will become a Lanmola from A Link to the Past, but is defeated in a different fashion. The Hookshot, Boomerang, Bomb, or Magic Rod will rid you of this form. It will change forms one last time.

Final Form: Dethl
The final form is the true form and will be the only new enemy you will encounter. Dethl will be in the form of a strange creature with two muscular arms extending from a center eyeball that looks a lot like Vaati. Using the Roc's Feather, Link must jump over the arms to avoid being hit by them. The center eye will be closed while the arms swing. Occasionally, it will open during the battle. Once it opens, attack with an arrow or the boomerang. This form will take the longest, but once it is over, the Wind Fish will awaken.

As well as being a possible inspiration for Vaati, Dethl also appears incredibly similar to a similar final boss in the Kirby series of games, also by Nintendo, and resembles the games antagonist "Dark Matter".

It takes sixteen hits to kill Dethl with the bow and arrows. However, a single direct hit (right in the center of the eye) with the boomerang will kill him instantly.

The Wind Fish
Once the Nightmare is dealt with, the Wind Fish will awaken. He will then tell you that Koholint Island was all a big dream, and when the raft carrying Link got struck by lightning, he fell into a deep sleep. Using the Eight Instruments Link gathered and playing the Ballad of the Wind Fish, Link and the Wind Fish finally awaken. Link awakens on a piece of his former raft and watches as the Wind Fish flies from overhead. The dream island of Koholint would only remain in his dreams.

Trivia

 * The Nightmare form of Ganon did not appear in the Link's Awakening manga.