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->''Seventeen years old. Youth. A magical time. All lies.''

What would you do if you thought you could save someone's life?

Would you do anything it took, right up to selling your soul to the Devil?

Diabolo follows the story of Ren and Rai, two teenagers who fell into the trap of the [[TitleDrop titular Diabolo]] and are now blessed with immense powers of offense and defense. The cost? Well, people gifted with the powers of Diabolo start to [[SanitySlippage go insane]] when they turn seventeen--[[BlessedWithSuck also the time their powers manifest]]--and surrender their souls at eighteen. Fully aware that there's little time left in their lives and the Diabolo ensnaring teens in their poisonous contract left and right, Ren and Rai devote themselves to saving those who have fallen.

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!!This manga provides examples of:
* AbusiveParents: Once Ren's mother finally reaches her breaking point, she realizes how much Ren is beginning to look like his father. [[spoiler: She then tries to [[RapeAsDrama rape]] [[ParentalIncest him.]] Failing that, she sets herself on fire.]]
** Mio had these as well and told Ren and Rai how her parents wanted her to disappear because she was [[IllGirl constantly ill.]]
* AerithAndBob: You've got your very simple Rai and Ren and Mio and ''then'' you've got the tongue-twistery generals: Nebiros, Satanachia, Sargatanas, Agliarept, Fleurety, and Lucifuge.
* AlcoholicParent: After the initial incident with Mio, Ren's mother became one of these.
* AlmostKiss
* AlphaBitch: Arisa from the 'Reincarnation Ritual' storyline.
* AmbiguouslyGay: Rai [[spoiler: and by that token, Nebiros as well.]]
* AnyoneCanDie
* ArcNumber: Seventeen is the primary one. It is the age when [[BlessedWithSuck powers granted by the Diabolo]] are at their strongest and also when most users begin falling into evil. It is also only the people who are this age who are contacted by Nema to join Heaven. Eighteen serves as a minor number in that it is the age when those blessed by the Diabolo forfeit their souls (if they haven't died yet) and, as is pointed out early in the series, one of the many ways you can get eighteen is 6 + 6 + 6.
* BigDamnVillains: Near the end, [[spoiler: Fleurety, Sargatanas and Agliarept do this. AND IT IS AWESOME.]]
* BittersweetEnding
* BlessedWithSuck: Both main characters have "Ultimate Offense" and "Ultimate Defense", at the cost of their souls on their eighteenth birthdays. Ouch.
* BodyHorror: The climactic roof rendezvous between Nema, the Master, Rai, and Nebiros starts off normally. [[spoiler: And then Nema and the Master's flesh starts melting off.]]
* BystanderSyndrome: See a potential rape victim? '[[HumansAreBastards Forget it.]] You don't want to get involved.'
* TheCharmer: Rai has hints of this and charms a handful of ladies through the series from a young girl to ''one of Satan's six generals''.
* {{Cloudcuckoolander}}: Rei.
* CloudcuckoolandersMinder: Nana fulfills this role for Rei.
* CompellingVoice: Satanachia not only is a master of this ability, but also thoroughly enjoys the [[InLoveWithYourCarnage inevitable outcome.]]
* CooldownHug: After going to the illegal hospital, this is invoked times two. It works. [[spoiler: But only on Ren.]]
* CoupDeGrace: Sargatanas does this to [[spoiler: Satanachia by connecting a dimension between her hand and Satanachia's heart and ''crushing it''. DAYUM.]]
* CrapsackWorld: The first storyline is about [[spoiler: a dead girl who was brutally raped and murdered by her boyfriend and then revived to bear a demon child.]] Most of the story is equally grim.
* CreepyChild: Satanachia [[spoiler: and Mio especially, once you find out what she has done.]]
* CursedWithAwesome: The powers of Ren and Rai are either this or BlessedWithSuck.
* DarkMagicalGirl: Fleurety
* [[spoiler: DeadAllAlong: This series likes this trope. Not only was Chiaki from 'Conception' murdered, but Mio from 'Omen' as well.]]
* DealWithTheDevil: The basis of the entire plot.
* [[spoiler: DownerEnding: Depends on how optimistic you are with the outcome of the story.]]
* DespairEventHorizon: Tsukiko quietly accepting [[spoiler: being kept as a sex pet and getting gang-banged by her roommates acts as this for Rai.]]
* DomesticAbuser: Several characters have these in their lives, including Mio Hashiguchi (her brother), Ren (his mother), and Tsukiko (''everyone'').
* DudeLooksLikeALady: Rei. His first appearance has random passersby wonder what his gender was.
* [[spoiler: FalseInnocenceTrick]]
* FetusTerrible: Chiaki's impregnated with one of these in 'Conception'.
* FingerLickinEvil: Fleurety pulls one of these in her debut after decapitating one of her bedmates.
* FourEyesZeroSoul: Nebiros
* FromBadToWorse: Oh, so you're doomed to lose your soul when you turn eighteen but you get [[CursedWithAwesome super awesome powers]] from it, well that's not so--wait, you start to [[SanitySlippage lose your mind]] as your [[WithGreatPowerComesGreatInsanity powers get stronger]], okay that's pretty bad, but still--you got your powers because the [[ChildhoodFriend little girl you played with]] all the time was sacrificed in a demonic ritual that you were blamed for, that's kinda screwed up--[[spoiler: your best friend goes batshit crazy and joins the dark side, [[WitchHunt the whole country is paranoid]], there's a giant Satanic [[ReligionOfEvil cult of seventeen year olds on the rise]], parents are murdering their children out of fear, and the only way to stop it is by killing not only yourself, but your best friend as well.]] Well [[PrecisionFStrike fuck.]]
* {{Gorn}}: Hello, hospital scene!
* GratuitousSpanish: A diabolo is a child's toy. Diablo, on the other hand, is another thing entirely... [[spoiler: However, one could always argue that Diabolo is correct, given the ending.]]
* HeroicBSOD: Ren has one [[spoiler: after Rai disappears]].
* HeterosexualLifePartners: Ren and Rai...[[HoYay maybe]].
* HologramProjectionImperfection: Lucifuge uses a hologram which is prone to flickering when he gets angry to act in lieu of his real body.
* HomelessHero: Both Ren and Rai qualify, though Rai's been at it for longer.
* HumansAreBastards: Welcome to the [[CrapsackWorld world of Diabolo!]] Most of the population seems apathetic, socially deviant, or hyper-paranoid after Nema starts broadcasting her message to seventeen year olds. The decent humans in the series can be counted on one hand.
* IllGirl: Both Mio and Kyouya.
* InfantImmortality: Averted. [[spoiler: A baby is thrown to its death within the first chapter]], which gives you a pretty good idea how seriously the mangaka take AnyoneCanDie.
* InLoveWithYourCarnage: Satanachia, full stop. And Nebiros seems to fancy it as well.
* [[spoiler: TheManBehindTheMan: Mio is the BigBad.]]
* MeaningfulName: Tsukiko means 'moon child'. [[spoiler: Fleurety gets her powers from the full moon.]]
* MinorInjuryOverreaction: In 'Omen' a woman is hit by a car, but [[FridgeLogic only seems to have a hand injury.]] She immediately starts screaming for someone to call an ambulance.
* NamesTheSame: [[OnlyOneName Mio]] the dead little sister and Mio Hashiguchi. [[spoiler: The ''other'' [[DeadAllAlong dead little sister.]]]]
* NiceJobBreakingItHero: [[spoiler: After meeting Rei, Rai gets horribly sick. Ren leaves to go find Nana because she knows where illegal hospitals for the homeless are. Fearing that it's a Diabolo trap, Rai chases after him and the two find the hospital staff brutally slaughtered. The smell of the blood and the sight of the mutilated bodies causes Rai to start his fall and leaves teetering on the brink of becoming [[SuperpoweredEvilSide Nebiros.]]]]
* TheNoseKnows: Rai, who is mentioned to have a good sense of smell several times. [[spoiler: It plays a part in initiating the fall.]]
* OffModel: Suddenly, Rai's missing his mouth on a splash page. Which is weird, considering that it would take ''two'' artists to overlook it. (The series is drawn by a pair of manga-ka twins.)
* OffWithHisHead: Fleurety performs one of these as her entrance to the series.
* PinkySwear: Ren and Rai share one of these in 'Awakening'. It helps Rai remember the warmth of their friendship and what he's fighting for. [[spoiler: And later Rai goes full Nebiros and ''bites his pinky off''.]]
* ThePromise: And it's a ''heavy'' one. Both Ren and Rai have promised to kill each other before they turn eighteen and fall to the Diabolo.
* RapeAsDrama: Chiaki and Tsukiko.
* RedOniBlueOni: Ren is red to Rai's blue.
* ReligionOfEvil: Heaven is not only most decidedly a cult, but an interestingly high-profile one. But then again, if a bunch of seventeen year olds started murdering everyone else and committing various other felonious acts, then someone's bound to take notice.
* RichBitch: Lucifuge acts as a male version of this.
* SelfMadeOrphan: [[spoiler: Mio]]
* ShipperOnDeck: Kyouya has his moments and is the only character who really comments on Ren and Rai's [[HeterosexualLifePartners interesting]] [[HoYay relationship]].
* ShownTheirWork: While the Six Great Spirits' names and abilities may seem sort of random, further research will yield that they're the six lead members of Hell's armies in the grimoire ''Le Dragon Rouge''.
* ShutUpKiss: Ren delivers one of these to Chiaki in 'Conception'.
* SuperpoweredEvilSide: Nebiros.
** All of the Great Spirits to varying degrees.
* SuperpowerLottery: The six Great Spirits. Agliarept can find out any secret, Satanachia is a puppet master who is also skilled in the CompellingVoice, Sargatanas can go ''anywhere'' by connecting two dimensions, Lucifuge is a TechnoWizard, Nebiros controls premonition and pain and is the best {{Necromancer}} in Hell, and Fleurety is [[{{Immortality}} the immortal queen of the night.]]
* SweetPollyOliver: Nana dresses as a thuggish boy at night so people don't bother her.
* SdrawkcabName: What's Nema backwards?
* TakingYouWithMe: Agliarept is taking the world with him, Nebiros and Ren are going down together, parents and children are committing murder-suicides across the nation... There's lots of this, especially in the final volume. [[spoiler: Oh, and Mio.]]
* {{Tearjerker}}
* WoobieDestroyerOfWorlds: All six of the Great Spirits.