Nightmare

The Nightmare is the name of both the last dungeon and final boss of Link's Awakening (though not the primary antagonist).

He is a servant of the Annoying Owl and seeks to destroy Marty before he can wake the Wind Fish. He accomplishes this by turning into a puddle and chasing Marty around a maze of dungeon rooms.

The Nightmare cannot be destroyed in the room where he appears. He has to be led into a room where he is vulnerable, so the first part of the fight consists mainly of running away. Once the player finds the right room, he will take on multiple forms. The first stage is the Skeleton. In this form he just hops around the room and must be dusted with Magic Powder to damage him. The second form is the Total Agahnim Rip-off. In this form he rips off Agahnim and launches glowing balls at the player that have to be batted back. If you have ever played a single Zelda game, or even just seen someone playing one while you were a baby, you've already seen this gimmick umpteen times.

The third form is Caterpie, who is totally the most intimidating Pokemon.

The fourth form is Gelnon, who is Ganon with the head of a Gel. He throws his gigantic double-edged halberd, but like everything else in this game is not too dangerous.

His fifth and final form is Ol' Skeleton-Arms. This is a giant eyeball with skeletons for arms. He represents the designers completely running out of ideas at this point and just drawing sprites at random.