Dream Eater

The Dream Eaters (ドリームイーター) are a species of creatures that debut in Kingdom Hearts 3D: Dream Drop Distance as a key element in the game. The Dream Eaters are forms that darkness takes when it enters the Realm of Sleep, and so come in two different varieties: the malevolent Nightmares, which eat good dreams and plant bad ones, and the acquaintable Spirits, which only eat bad dreams. Spirits serve as supporting characters that can be tamed by the player while Nightmares are the primary antagonists in the game.

Physical appearance
The Dream Eaters are designed after various animals and come in various shapes and colors, more so than the Heartless or other beings of darkness.

To distinguish the two varieties, the two types of Dream Eaters are characterized by their emblem, which signifies whether it is a Spirit or Nightmare, and their color schemes; Spirits have light, pastel color schemes while Nightmares tend to have darker ones. Spirits' eyes are also black with yellow star-shaped pupils, in contrast with the Nightmares' eyes which are round, red, lack pupils.

Nature
Dream Eaters exist only within the Realm of Sleep, feeding on the dreams and nightmares of those who sleep. The behavior of Nightmares is typical of darkness entities, being violent and aggressive, although much more expressive than the the series' previous enemies.

Unlike most creatures made of darkness, Spirits are the polar opposite; they are benevolent and playful, and are willing to assist in any way they can to people who are kind to them. In return, Dream Eaters can share power with one another and with other beings in a variety of ways. Their nature indicates a positive form of darkness often misinterpreted by most enemies that use darkness, in that darkness by default is not a malicious presence despite its appearance.

Kingdom Hearts 3D: Dream Drop Distance
The Dream Eaters appear in the game as the natural embodiments of darkness in the sleeping worlds, worlds that were previously attacked by the Heartless but remained within sleep instead of returning to the Realm of Light. These creatures were born following the defeat of Ansem, Seeker of Darkness, where the Nightmare Dream Eaters took its Heartless predecessor's role in seeking out the world's keyholes, then promptly consuming it. Sora and Riku encounter the beings while carrying out their Mark of Mastery exam, and with the help of the Spirits and the defeat of the Nightmares, manage to awake the sleeping worlds. Eventually, it is revealed that Riku is within Sora's own dreams since the beginning of the test as a Dream Eater himself. To save Sora from slumber, Riku absorbs all of his Spirits' power in a battle against the Nightmare possessing Sora and wins, allowing Sora to wake up. Afterwards, Sora decides to visit the sleeping Traverse Town again, and he is reunited with all of his Spirit friends he had made on his journey.