Nightmare

Nightmare, often known collectively as Dethl, is the final bosses of The Legend of Zelda: Link's Awakening. It is hidden inside the Wind Fish's Egg. To reach Nightmare, Link must enter the Egg after collecting each of the eight instruments from the dungeons of Koholint Island.

The exact nature or creation of this evil being is vague at best, but its goal is clear. It put the Wind Fish to sleep in order to keep it locked in a tormenting nightmare, and for the island of Koholint to retain its existence so that it may rule it itself. Nightmare presumably has the power to take the shape of any foe encountered in Link's past, including his most fearsome foe, Ganon. It can, however, only take one physical presence at a time. Its "Dethl" form is the only original shape the Nightmare takes, suggesting that perhaps, it is its true form.

Location
Once inside the egg Link must drop down into a maze like area, and navigate it to the end by following one of three randomly selected paths, which can be found in a book in Koholint Library once the Magnifying Glass is obtained. At the end of the path, Link then reaches a hole to jump in to, leading to the final battle.

Battle
The Nightmare comes in six different forms, each appearing as soon as Link slays the previous one. The six forms are:

Giant Bit
Giant Bit is a giant blob that hops around the room. As its name implies, it is a large Bit. To defeat this form, Link must sprinkle Magic Powder on it multiple times.

Agahnim
The Nightmare's Agahnim form is not the real ''A Link to the Past villain, but merely the Nightmare made to look like the evil wizard. Unlike the original Agahnim, this nightmare form has oddly jointed fingers and huge white eyes. To defeat this boss, Link must hit his energy balls back at him, much like the real Agahnim.

Moldorm
Moldorm is similar in behavior and appearance to the Moldorm from Tail Cave. Link attacks it the same way, by hitting it in the tail, but in this fight, he must hit it more times and faster. As characteristic of all forms of Nightmare, Moldorm has two huge white eyes. Despite the fact that there is another Moldorm in the game, this Moldorm more closely resembles the attack pattern of the Moldorm in A Link to the Past.

Ganon
As with Agahnim from A Link to the Past, this is not the real Ganon, but the Nightmares made to look similar to him. He is depicted as a creature twirling an axe-like trident above his head while flaming bats are conjured up to fly at Link. Ganon then attacks by throwing his double-sided ax. The easiest way to defeat him is to charge at him with the Pegasus Boots and sword, but the Spin Attack works as well. The fight is similar to the Ganon fight from A Link to the Past.

Lanmola
Lanmola is a fast moving blob that bumps into Link to attack him. Despite being called a Lanmola, it behaves very differently from real enemy. To defeat Lanmola, Link needs only hit it once with the Hookshot, Magic Rod, or Boomerang.

Dethl
Dethl is the final form of Nightmare and the final boss in Link's Awakening. It is considered the Nightmare's "true" form.

Fans have noted that Dethl may have inspired the design of Vaati, the main villain in The Legend of Zelda: The Minish Cap, The Legend of Zelda: Four Swords, and The Legend of Zelda: Four Swords Adventures.

Once Nightmare transforms into its true form, Dethl, equip the Roc's Feather and the Bow. Stay in front of its eye at all times, however, and be careful not to ram into it as it will damage Link. Dodge its spiky arms with the Roc's Feather and once it opens it eye, shoot at it with the bow until it closes its eye again. Dethl will move faster and faster the more damage will be done to it, so pay attention to the arms. After it takes about twenty hits, it will die.

Strangely, if the Boomerang hits Dethl directly in the eye, it is a one-hit kill and the battle is over.

Manga
"We are born from nightmares... In order to maintain order in this world enclosed in a dream while the 'Wind Fish' was given eternal sleep... We were born to prevent the awakening, because the island will disappear... WE ARE GOD!!!" - Nightmare

In the manga, Nightmare is for the most part the same, save for some different dialogue. They refers to themselves as a god, and that they are responsible for preventing the end of "their" world's existence. It is unknown whether or not there is any truth behind their claim of godhood, as they may indeed collectively be the Deity of Nightmares, or they simply are conceited by the power they possess.