Shadow Nightmare

The Shadow Nightmares is the collective name of the various final boss forms in Link's Awakening. The final Nightmare, seemingly a collective of multiple Nightmares, 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. The Nightmares are the primary antagonists in the Wind Fish's dream on Koholint Island, and are the force single-handedly responsible for the forced, continued slumber of the Wind Fish. Like many of the bosses Link fights in the game, the Shadow Nightmares are self-aware: at the beginning and end of the battle, it reveals the plans to continue the Wind Fish's slumber, and speaks of how its death will signal the end of "its world", respectively.

Giant Zol
The first shadow is also known as Giant Gel, and is a giant Zol, a common enemy throughout Link's travels on Koholint and his previous adventures. The Giant Zol bounces after Link much like a normal Zol, and occasionally disappears into the ground only to appear elsewhere. While not particularly danagerous, as first the Giant Zol can seem impossible to destroy; striking it with his blade will simply cause it to shatter and reappear elsewhere. However, if Link simply sprinkles Magic Powder on it three times, it will be defeated and proceed to morph into its next form.

Agahnim's Shadow
The second form is a shadowy version of Agahnim, the evil sorcerer from A Link to the Past. This form can be defeated in the same manner as the true Agahnim, by playing a grueling game of Dead Man's Volley. Link must parry balls of magic back at Agahnim to cause damage. Similar to the true Agahnim battle in A Link to the Past, some blasts of magic cannot be reflected, and will instead scatter, sending smaller bolts across the arena. Agahnim will however not use his lightening attacks or attempt to create fake copies of himself, nor will Link take damage from striking him with his sword like he did in the previous game. After hitting him with his magic enough times, he will spin on the spot before breaking apart and moving to the next form.

If timed properly, Link can also use the Shovel to deflect Agahnim's blasts 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. The only changes are that there are no holes to fall down, and Link must strike the tail many more times than the original Nightmare he first fought in the Tail Cave. 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
Despite being known as Lanmola, this particular form does not have much relation to its namesake; instead it appears to be the collective form of the Shadow Nightmares that Link sees inbetween each form, making a last ditch attempt to defeat Link. It is seemingly invulnerable to Link's sword and will home in on him, leaving trails of itself behind as it chases him around the room. It can be defeated with a powerful strike, preferably from the Magic Rod, but a Spin Attack, Hookshot, Bomb or the Boomerang will have the same effect. A single strike is all that is requires to stop its charge and cause it to reveal its final, deadly form.

DethI
The final form is the only new enemy encountered in this battle, DethI ("Deth I", also known as Death Eye). DethI consists of a large pulsating mass with a single, usually closed eye from which two dark, shadowy arms with huge spiked tips extend outwards. Using the Roc's Feather, Link must jump over the arms to avoid injury once they begin to swing and avoid the movement of the central body. Occasionally, the eye will open during the battle, upon which Link must strike it using the Bow. After depleting half of its health, Link's deadly game of jump rope will get harder, as the swinging arms speed up and start moving in a less obvious pattern, with one arm moving faster than the other. It will take a long and challenging battle, but sixteen direct shots in the eye with the Bow will finally defeat it. This form will take the longest, but once it is defeated, the Wind Fish will awaken.

DethI can also be defeated by using a secret method; if Link has located the powerful Boomerang, he can defeat the final shadow with a single hit to the eye. The hit needs to be direct, however, so sometimes will not work.

Aftermath
Once the Nightmare is dealt with, the Owl will speak to Link and a stairway will appear. Link travels up the stairs to speak to the Owl a final time, before the Wind Fish appears. He will confirm that Koholint Island is a dream world, and that Link ended up in his dream after his ship was struck by lightning while he was sailing home. 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 ship and watches as the Wind Fish flies overhead. The dream island of Koholint would only remain in his memories.

Trivia

 * The Giant Zol is very similar to the form taken by the Giant Buzz Blob once you sprinkle Magic Powder on it. This monster that was added to the DX version of the game, and was possibly included to create a reason for this shadow's appearance as a representation of a past battle, or to simply give the player a clue for how to defeat it.
 * DethI resembles Vaati's final form in Four Swords and The Minish Cap, and was possibly an inspiration for his design.
 * A tattered poster in Ocarina of Time's Shooting Gallery bears some resemblance to DethI, though there is not a proven connection.
 * 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. Both Vaati and Dark Matter did not appear until long after the first release of Link's Awakening.