You know how sometimes you're listening to a pretty chipper song, and you think it's gonna stay that way? Sometimes it doesn't. That's what you call the descent into darkness. A Descent Into Darkness Song is a song that changes its tone, slowly or quickly, into something much darker. The evolution of the music can involve modulating from a major key to a minor one as shorthand for a darker style.
If there are words which are consistently upbeat, there may be Lyrical Dissonance at the end; the inverse is also true if they're depressing throughout it. If in a musical, it can be a positive song that becomes sinister when the heroes leave. If the song is wholly cheerful but later sung tragically, you have a Dark Reprise. This can be a form of Villain Song.
- "Princess of December" from UFO Ultramaiden Valkyrie. Starts out as a romantic song of eternal love and devotion, sung by the titular Valkyrie, only for it to be hijacked halfway through by her Shadow Archetype, Valkyrie Ghost, who turns it into a heart-wrenching song of loneliness and despair. It's one of the first hints that the Dark Valkyrie is actually a tragic figure, rather than the out-and-out villain she first appears to be. You can hear it here.
- Doki Doki Forever, a fan-made Doki Doki Literature Club! song starts off sounding very happy and upbeat, with very sweet, almost saccharine, lyrics, reflecting how the game is initially a very sweet dating sim. However, the song gets progressively darker starting from the second verse, very much like how the game gets progressively darker in the second act. The song remains upbeat, but gets more distorted and glitchy-sounding as it progresses, and there are also jumpscares and sudden cuts to disturbing images from the game, such as Sayori's suicide, which get more frequent as the song goes on. The lyrics also become more disturbing and often have dark double meanings, such as Yuri's line 'Will you make the cut?" also referencing her self-harm.
- "The Moon Rises," based on My Little Pony: Friendship Is Magic, is supposed to serve as a Villain Song for Nightmare Moon. It begins with Princess Luna rhapsodizing about the beauty of the night, but slowly becoming more bitter until she decides to make it last forever.
- In Friendship is Witchcraft Pinkie Pie's orphanage song does this. It begins as a happy song, but then Pinkie starts singing about how unworthy she is.
- Terminator 2: Judgment Day: A very subtle, very quick version occurs during the mall confrontation scene where both Terminators find John at the same time. Because the movie is playing up the fact that either Terminator could be evil, you hear both of their themes play one after the other. First Arnold's strong metal percussion then the T-1000's low ominous moan. Then, as both of them meet each other with John in the middle, for a brief moment both play on top of each other, adding to the tension of which (if not both) Terminators are evil. Then Arnold saves John and the -1000's low moan completely takes over. He's our bad guy.
- In South Park: Bigger, Longer & Uncut, "Hell Isn't Good" starts out with Kenny, who died in an earlier scene, making his way to Heaven, with hopeful lyrics underscoring how Kenny is finally at peace. But when he finally reaches Heaven, the gates reject him and the heavy metal guitar kicks in as poor Kenny falls screaming into the bowels of Hell.
Little boy, you're going to HELL!
You said bad words,
threw rocks at the birds,
and now this is your hotel!
You aint goin' back!
This ain't Disneyland!
- Inverted in The Nightmare Before Christmas—"Jack's Lament" starts as a slow, somber song, with the title character asking "What Have I Done?" About halfway through, however, he becomes bitter and then, quite suddenly, cheerful, realizing that his failed experiment still managed to get him out of his existential funk.
- The Hunchback of Notre Dame, Frollo's "Hellfire" song counts as a Dark Reprise of "Heaven's Light", but it also counts alone as this trope. A sacred chant leads into the song, With composure, Frollo sings to the Virgin Mary, "You know I am a righteous man..." and then the song gets darker as he sings of how desire for Esmeralda tempts him. The accompanying chants turn ominous, and Frollo pictures himself condemned. He concludes: "She'll be mine or she will burn!"
- The Little Mermaid's "Poor Unfortunate Souls" could perhaps count. It begins with Ursula trying to play herself off as innocent, but she becomes more Obviously Evil (not to mention bombastic) as it goes on.
- Tangled has Mother Gothel sing "Mother Knows Best" to Rapunzel, which was a wacky Broadway type song, but had insulting lyrics to lower Rapunzel's self-esteem and make her more dependent. However, later in the film when Rapunzel is becoming more independent, and refuses to return to the tower with Gothel, Gothel's reprise of her song noticeably takes a dark turn as she becomes more hostile and insults Rapunzel further.
- Ennio Morricone's main theme for Adrian Lyne's 1997 adaptation of Lolita starts off sounding like a standard soundtrack for a romantic drama, but the notes become increasingly downbeat and off-key as the music goes on. It does the job of contrasting Humbert's fantasies about Lolita with the twisted reality of their relationship.
- The Once-ler's Villain Song from The Lorax gradually becomes darker as the song goes on, starting in his casual worker clothes with his acoustic guitar, to wearing a flashy new business suit, to making huge, polluting factories, to not caring if "a few trees are dying", while you see him destroying said trees with the soundwaves from his new electric guitar, and carelessly hacking down every tree left and right while the forest creatures run for their lives. Not only does the song get so much darker, his voice gets a distinct malevolent echo at the end.
- "The End", by The Doors, famously starts off as a ponderous musing about the nature of finality. However, eventually the imagery shifts into describing a masked killer grappling with the desire to kill, and then killing, his own parents. Afterwards, the tempo builds up to the end, creating a mesmerizing climax. Which, incidentally, makes it the perfect song to be used to bookend Apocalypse Nowa descent into the evil that lurks in the human heart.
- Bill Bailey has an OTT Obsession Song that starts a bit depressed. Only a bit, mind you. Then it goes serenely upbeat and joyous, followed by being unbelievably angry and unhappy.
- The Flight of the Conchords song "Petrov, Yelyena, and Me": The lyrics get more disturbing as the singer catches on that Petrov and Yelyena are eating him piece-by-piece.
- Several songs by Stephen Lynch. "Superhero" starts with describing various superheroes kids might want to be before turning into a diatribe against his unfaithful wife who now lives in Hawaii with a doctor, dumping him with three kids and no job. For those interested — song here.
- An instrumental example, Rude Awakening, No. 2 by Creedence Clearwater Revival starts out mellow but becomes more and more spooky as the song goes on.
- The Vocaloid song "Wordplay" in the Evillious Chronicles by mothy. It begins with the singer talking cheerfully about the first time she learned of the world, friendship, and love. Then it slowly cycles through the first time she learned of pain, punishment, lies and evil, with the video getting darker and grayer as well. It's implied this is the same person who becomes a Mad Scientist in the later song "Moonlit Laboratory".
- Kendrick Lamar's "i" starts out as a comparatively upbeat song about the viewpoint character staying hopeful in grim situations, but it doesn't stay that way. By the end, he's given in, although he does express hope that someone will learn from his mistakes. The big hint that it's moving into significantly darker territory comes when the hook drops in pitch for a deeper, more ominous sound.
- Helen Reddy's hit Angie Baby (written by Alan O'Day) which starts out okay, being about a "touched" girl who uses her radio to keep her company. It soon turns to the girl having to be removed from school and eventually to the subject of potential rape and the vague aftermath that results in the disappearance of the young man, leaving the listener to wonder whether he was killed by Angie with something maybe worse (since Angie has "a secret lover who keeps her satisfied") or she's just imagining him still there. Helen Reddy and singer/songwriter Alan O'Day (Undercover Angel) eventually revealed the truth: Angie was a Reality Warper, and the boy with 'evil on his mind' was not the first she shunted away for her pleasure.
- The song "Handlebars" by the Flobots starts with a young boy innocently bragging "I can ride my bike with no handlebars." As the song continues, the boy grows up and his statements become darker and more grandiose, ending with "I can end the planet in a holocaust."
- "More Gun" from Meet The Engineer? It starts off as a calm campfire song but VERY quickly turns harsh and ends up sounding like a western duel song. If you are interested, here it is.
- The Legend of Zelda:
- The Legend of Zelda: Majora's Mask has the Clock Town theme. It starts out rather upbeat enough, as the town is mostly going about its business and the townspeople blissfully ignorant that anything's wrong. Sure, the moon is up there glaring at them, but it's way out there. What's it gonna do? The next day, the moon is demonstrably closer than its original position, and that night, closer still. The music is slightly darker to accommodate this. Then, by the third day, the music is outright apocalyptic, since the townspeople have all but fled or given up in terror, and the moon is so close it looks like it could cleanse the planet with one strong snort.
- The Legend of Zelda: Twilight Princess has Hyrule Castle theme, which is the final dungeon of the game. It starts off soft and majestic, with a revision of the Hyrule Castle song from The Legend of Zelda: A Link to the Past. As you progress through the dungeon, however, the song gains a bassline that may sound...a bit familiar. Later, the bassline is seamlessly expanded into Ganon's theme, which is overlaid onto the Hyrule Castle theme. From that point on, Ganon's theme slowly takes over, becoming louder and more pronounced while the Hyrule Castle theme becomes softer and more subtle. By the time you're in the stairway before the final battle, Ganon's theme has completely taken over.
- The Legend of Zelda: A Link Between Worlds did the same thing as Twilight Princess. Lorule Castle's theme adds more and more to it as you progress, but as you get closer to the end, you hear Ganon's theme as a bit of a bridge in the tune.
- In EarthBound, the infamous Final Boss theme "Cease to Exist" (better known as "Pokey Means Business!", Gigyas's first theme) starts off predictable and vaguely melancholic, then abruptly switches to frantic and disturbing.
- From Portal 2, the track 'I AM NOT A MORON!' fits this trope to a T: it starts out with triumphant, happy electronic tones, but halfway through it switches to a much more sinister tone played by brass and strings, culminating in a dark reprise of the turning point in the track, combining both orchestra and electronic music, ending in a much different place than it started.
- "I Believe" from Spring Awakening has its tone darken as the two leads transition from kissing to ambiguously consensual sex.
- Merrily We Roll Along has three songs about the increasingly strained relationship between Frank, Charley and Mary. In increasing severity and in chronological order (which is more or less the reverse of their order in performance):
- "Growing Up" begins as a pleased soliloquy at the piano by Frank about his two friends, but has a markedly more dissonant bridge where he voices his frustrations about them.
- "Old Friends," a trio in which Frank, Charley and Mary act as Vitriolic Best Buds, with the vitriol contained in a troubled bridge section which descends into a heated argument between the three. The tune of this bridge is earlier reprised with different lyrics as "Like It Was" following a Dark Reprise of the refrain.
- "Franklin Shepard, Inc." (which begins with the same diegetic piano flourish used in "Growing Up") starts out with Charley in a relatively cheerful though sarcastic mood, but the music stops and restarts wearily, then frantically as he comes to the realization that he's been Saying Too Much and their longtime friendship is no more.
- "Our Love Is God" from Heathers starts out as a sweet and romantic duet between Veronica and J.D. and transforms into a delusional J.D. singing by himself as he murders Kurt and Ram.
- In Turnabout Musical, Phoenix's 'Reawakening' song starts out as upbeat as could be, as he's feeling good about his victory in court. It turns downright heartbreaking when he returns to the office to find his mentor murdered.
- In Tangled: The Series, one of Rapunzel's friends, a young inventor named Varian, is determined to make his father proud of him, which is also the title of his song. However an accident in his lab traps Varian's father in crystal. Desperate for help, Varian goes to Rapunzel, who is swamped with a natural disaster and other responsibilities for her kingdom, and she sends Varian away. Feeling both bitter and guilty over this accident causes Varian to have a noticeable FaceHeel Turn in the middle of his originally chipper song, ending with a clenched fist, and with a sinister tone where he proclaims that "they will pay".