Nightmare

The Nightmares are the main antagonists of. They are monsters that invaded the Wind Fish's dream world of Koholint. They seek to rule the island by making the Wind Fish sleep endlessly, and they attempt prevent Link from obtaining the Eight Instruments of the Sirens which can be used to wake the Wind Fish. To that end, eight Nightmares guard each of the instruments in deep monster-filled labyrinths found on the island. Unlike other boss monsters in the series, which are primarily wild beasts, these creatures are more intelligent, able to speak to Link. They refer to him as the outsider, and even go so far as to reveal the truth of the island to him in an attempt to stop him from waking the Wind Fish.

To fight each Nightmare, Link must obtain a Nightmare's Key before entering the Nightmare's Lair.

While only the eight main boss monsters and the Shadow Nightmares are referred to as Nightmares, other monsters on the island become more active once Link arrives, and are found throughout the dungeons and world of Koholint. The Grim Creeper, despite not being the main boss of the Eagle's Tower, rides the Evil Eagle and speaks to Link, and is thought to also be a Nightmare himself.