There are many different kinds of dreams: sweet dreams that make you happy, nightmares that scare you, weird dreams that leave you confused, prophetic dreams that supposedly predict the future... However, many works of fiction that focus on dreams cut it down to good dreams and nightmares.
Often, this trope involves two main groups or individuals; they may be different gods if indivuals, different species or kingdoms if groups, etc. One is based on the concept of good dreams. As the good guys, they are likely the ones who create or inhabit these dreams, and generally want you to sleep tight and have happy dreams of riding magical unicorns in a colorful land made entirely of candy and pastries, or flying freely through the sky and cosmos as Superman.
Then there's The Dark Side, which is based on nightmares. The dark creatures of the nighmare world, often led by a Nightmare Weaver, only exist for the sole purpose of dragging you into a state of seemingly endless torment the moment you fall asleep, and are portrayed as Always Chaotic Evil. They will attempt to make sure that you'll drown in a pool of thick, black oil with no hope for escape, that you'll always have legs made of Jello and filled with syrup as you helplessly stumble around like a teenager in a slasher film, unable to gain distance between you and the teleporting Monster Clown approaching closer and closer. And pray to God he isn't hiding under your bed or in your closet every night, or even worse, that you don't die in real life if he catches you in your nightmares...
Thus, the two opposites are fundamentally eternal enemies, and will battle it out for the fate of humanity's sleep, where the good guys work to foil the bad guys' plans as well as defend people's, but most commonly children's dreams from nightmares, while the bad guys seek to eradicate their sole obstacle in the way of their goal to spread endless nightmares and fear all over the planet. Expect the latter to play the role of the creepy villains in a sugary, happy work, and bonus points if the Evil Overlord is the Boogeyman himself.
The Dream Weaver and The Sandman may also show up to try to influence people's dreams, and the latter is likely to butt heads with the Boogeyman at some point. A Dream Walker and Dream Spying may be in play as well.
Contrast Real Dreams Are Weirder.
- In Little Nemo In Slumberland, the good kingdom of Slumberland (ruled by King Morpheus) is threatened with mischief by The Trickster Flip. Slightly averted in that Flip is not a nightmare as such but the nephew of The Dawn, who causes Slumberland to disappear each morning and all visitors to wake up. However, Flip causes more than his share of chaos and eventually has his uncle destroy Slumberland permanently (it gets better).
- Played with in The Sandman's "Doll's House" arc, in which Dream chases down a bunch of sentient dreams and nightmares that got loose during his confinement. On the one hand, his capture of Brute and Glob and the Corinthian are unambiguously good things, as they are nightmares who have created terrible problems in the waking world. On the other hand, Fiddler's Green is a perfectly harmless dream who just wanted to experience the physical world and didn't see the harm in doing so while his master was away, and yet at first Dream thinks of him as no different to the nightmares, and only sees the insult to his authority rather than the suffering they cause. That said, discovering that Fiddler's Green is Gilbert who is not only kind and generous, but even willing to sacrifice his own life for a human, prompts Dream to be merciful, and is the start of his Character Development into a more caring individual.
- Brute and Glob come from the seventies version of The Sandman, where they were presented as "the baddies" in young Jed Walker's dream world, regularly defeated by Garret Sanford (and later Hector Hall). This got deconstructed in the Gaiman version, which revealed Brute and Glob had set the whole scenario up themselves in the boss's absence in order to provide a regular source of dream energy. Gaiman's Dream has no problem with nightmares as such; after all, he created them, and the terror they create in the Dreaming is, in theory, serving a purpose. Unfortunately, the more powerful ones have a tendency to go beyond their remit.
- Zig-zagged in the cosmology of The Multiversity, which presents Dream and Nightmare as opposite each other in the Sphere of the Gods, in the same way as Heaven and Hell or New Genesis and Apokalips. However, unlike those realms, they do not seem to be opposing each other in an "eternal war" sort of sense, with the text describing Nightmare as simply the flip side of the Dream King's realm.
- In Rise of the Guardians, Sandy creates good dreams for children and Pitch Black creates nightmares for them instead. They use sand-like horses to make it work.
- In Twice Upon a Time, dreams are created in Frivoli, a candy-colored land of whimsy, and delivered by Greensleeves and his Figmen of the Imagination. Nightmares, on the other hand, are produced in the Murkworks, an ugly, smoke-belching factory, by Synonamess Botch and delivered via bombs by vultures. The plot involves Botch kidnapping Greensleeves and the Figmen as part of his plan to give the world endless nightmares.
- In A Journey to the Land of Dreams by Dmitry Drimov, the Big Bad rules over nightmares and drug-induced hallucinations, while the heroic characters from the Land of Dreams can conjure good dreams (or, more often, the dreams in the Land conjure themselves). There is also the neutral and nameless world of small icebergs that carry mundane everyday dreams. Interestingly, the villain is aware that his domain isn't very pleasant, but his planned solution (summed up roughly) is to steal all the colors from the Land of Dreams to make the nightmares prettier.
- In Roverandom, the Man on the Moon is responsible for good dreams and the moon-dwelling Giant Spiders for the bad ones. Since they're all frightened of the Man, they make nightmares in secret and when they are sure he isn't watching.
- In Nomine: Beleth is the Demon Princess of Nightmares. She and her demon underlings try to get humans to choose the path of evil by sending them terrifying dreams. Blandine is the Archangel of Dreams. She and her angelic servitors send pleasant dreams to inspire hope and ease stress, thus encouraging people to act in a positive manner.
- Bloodborne is a Lovecraftian take on this trope: you play a Hunter of Monsters based in the Pocket Dimension of the Hunter's Dream, who invades the pocket dimensions created by the nightmares of undead Eldritch Abominations (one in the main game, and a different one in the DLC), ostensibly, to stop the curse they put over Yharnam. However, while the Hunter's Dream looks, at first glance, like the Good Counterpart to Mergo's and Kos' nightmare worlds, it is eventually revealed that it is likewise a domain of an Great One, the Moon Presence (or the Doll, depending on your interpretation of the game), who just happens to favor peace and quiet, unlike the other two, but is otherwise just as removed from human morals as them.
- Dreamfall Chapters plays with the trope. Lux represents the dreams and Baba Yaga the nightmares. However, Baba Yaga is actually necessary for the dreams, thus life, to exist. The real antagonist is the Undreaming, which wants to destroy all dreams.
- EverQuest: Among the pantheon of gods exists Cazic Thule, the evil God of Fear. Cazic has two children who are each gods themselves. Terris Thule is the evil Goddess of Nightmares, who takes after her father. Morrel Thule is the God of Dreams, and is considered the Black Sheep of the family for inherently being a good-aligned god.
- In Fire Emblem Heroes, this is the very premise of Book IV involving the álfr, with the ljósálfr as the Dream Weavers while the dökkálfr are the Nightmare Weavers.
- In Kingdom Hearts 3D: Dream Drop Distance, Dream Eaters are divided between Spirits, which eat nightmares and can help Sora and Riku in their quest, and Nightmares, which eat good dreams and act as enemies.
- Kirby's Adventure: The conflict begins when King Dedede takes apart the Star Rod, a magical item that creates dreams for the residents of Dream Land. Kirby must reassemble the Star Rod so that everyone can have dreams again. But when he does, he accidentally summons a demonic being called Nightmare, who wants to spread nightmares across Dream Land. Only once Kirby destroys Nightmare does Dream Land go back to having happy dreams.
- Klonoa is a creature who travels around dream worlds to save them from various villains, some of which are stated to be nightmares. For example, the evil Ghadius in Door to Phantomile plots to destroy Phantomile by summoning Nahatomb, an Eldritch Abomination literally made from nightmares.
Ghadius: Hear this, strange dream! The world is finished! And this is the beginning of the perfect nightmare!
- The Legend of Zelda: Link's Awakening: Given that the game is All Just a Dream, this is the case with the game's conflict. The various townsfolk and NPCs who help Link out (Especially Marin) fulfill the role of the dreams, whereas the monsters who oppose Link are the Nightmares. For that matter, the bosses are even called "Nightmares".
- NiGHTS into Dreams... zigzags this trope. Wizeman the Wicked is the leader and creator of the Nightmaren, an evil race of nightmare creatures, and he wants to take over the world. However, NiGHTS, the protagonist, is a Nightmaren themself, who turned against Wizeman, showing that the Nightmaren aren't all bad. However, the good dream creatures, the Nightopians, aren't really active at all in fighting Wizeman, leaving NiGHTS and the human children they team up with to do most of the work. Basically, this would be Nightmare vs. Nightmares, except NiGHTS is fighting on behalf of the good dreams.
- World of Warcraft has the gentle druidic Emerald Dream, that is under attack from the Old God-corrupted forces of the Emerald Nightmare.
- Adventures of Sonic the Hedgehog: In the episode "Boogey-Mania", the kindly Professor von Schlemmer invents the Dream-A-Ma-Jig, a machine that takes creatures from the dreams of whoever puts it on and brings them to the real world. Von Schlemmer demonstrates by summoning a Non-Ironic Clown. However, the evil Dr. Robotnik kidnaps von Schlemmer and uses the Dream-A-Ma-Jig on himself to summon a creature from his nightmares, simply called the Boogey Man, so it can terrorize Mobius. Sonic and Tails save the day by using the machine on Tails to create a stronger creature from his food-themed dreams.
- Captain Zed and the Zee Zone has Captain Zed and the Dream Police spending their nights guarding the dreamscape of children against the Nightmares.
- The Dreamstone: The heroic residents of the Land of Dreams create good dreams, while the Evil Overlord Zordrak wants to spread nightmares. Notably, the heroes treat dreams as Serious Business.
- Moondreamers, a segment on My Little Pony 'n Friends, has the eponymous Moondreamers in conflict with the nightmare-spreading Queen Scowlene.
- Potsworth & Company, AKA Midnight Patrol, was about a band of kids and their dog protecting the Dream Zone from the evil Nightmare Prince.