A long crossover fanfiction by [[http://www.fanfiction.net/u/2198988/Soraga Soraga]], ''[[http://www.fanfiction.net/s/5651315/1/Code_Geass_50k_Lelouch_of_the_Imperium Code Geass 50K: Lelouch of the Imperium]]'' is to ''Anime/CodeGeass'' what ''Fanfic/ShinjiAndWarhammer40K'' is to ''Anime/NeonGenesisEvangelion'', melding the series into the bleak world of ''{{Warhammer40000}},'' albeit with a dash of ''FanFic/TheShapeOfTheNightmareToCome''.

Set in the [[DarkerAndEdgier even more grimdark]] future of the [[BadFuture 51st Millennium]], [[VestigialEmpire the Imperium of Man]] is on its last legs as the the forces of Chaos move ever closer to Terra. Its once mighty power now reduced to a mere shadow and the Golden Throne itself breaking down, all humanity could do it seems is to delay the inevitable. That is, until hope springs from the Emperor's forgotten soldiers: the Black Knights, its immortal leaders bracing themselves for a day they've spent countless generations preparing for. But meanwhile, the once-great being formerly known as Lelouch Lamperouge struggles to stay alive against the machinations of both Tzeentch and the growing insanity of the Warp...

Currently at 25 long chapters (26 if one counts the prologue), the fic manages to mix the two series almost seamlessly. Noteworthy as well are the efforts put in not only [[ShownTheirWork showing ridiculous attention to detail]] but also adding surprises in the mix relating to both source materials while staying true to canon (except for a few key points relevant to the plot).

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Tropes contained include:
* TheAestheticsOfTechnology: Deliberately invoked, with the Black Knights' sleek, highly efficient technology shown as decidedly more advanced than the familiar bulky, techno-gothic trappings of the Imperium. Justified in part due to the Knights having retained all the progress made during the Dark Age of Technology.
* AncientConspiracy: The Black Knights had come a ''long'' way from its origins in the anime, with much of its existence prior to the Warp exile largely in the shadows, keeping watch over humanity's evolving civilization.
* AndIMustScream: The Emperor.
* AntiHero: Lelouch/The Emperor, although he clearly didn't like some of more questionable deeds done in his time.
* BioAugmentation: The [[spoiler:"modern" Black Knights]] are revealed to be a genetically modified subspecies that emerged under Lelouch's watch during the Dark Age of Technology. The fragmented records of their development were later on used to create the Primarchs and Space Marines, though some of the details were mentioned as having been made up to replicate the original results.
* CrapsackWorld: Is it ''ever''.
* CultureClash: Between the Imperials and Knights. Made more explicit by the Knights apparently preserving Earth's records and cultures much more accurately as well as retaining much of the Dark Age of Technology.
* DeathSeeker: It's speculated by the Guardian [[spoiler:aka Euphie]] that a part of the Emperor considers both his internment in the Golden Throne and the warped CrapsackWorld that his Imperium ultimately became as punishment for his sins.
* DesignerBabies: [[spoiler:Euphie]] reveals this to be the "dark, dirty secret" behind the "modern" Black Knights' origins.
* IDidWhatIHadToDo: It's shown in the flashbacks that the Emperor ''really'' didn't like some of the acts he and his Imperium committed prior to the Horus Heresy, wishing at times to rather avoid them if he could help it. This is tied however to the XanatosSpeedChess he's playing against the Chaos Gods.
* DownerBeginning: In the distant past, it's revealed that Zero Requiem [[spoiler:failed in the long run, resulting in Britannia's near destruction]]. Thousands of years later, there's also Lelouch/The Emperor's Imperium sliding into the [[CrapsackWorld grimdark mess]] people know and love...only to slide into ''further'' misery by the 51st Millennium.
* EndOfAnAge: In many ways. [[spoiler:Including the Imperium ''itself''.]]
* EnemyMine / WeAreStrugglingTogether: The growing power of Chaos was what motivated the weakening Imperium to drop its "[[FantasticRacism kill Xenos on sight]]" policy and side with the Eldar and Tau. It's about as stable and trustworthy as one would expect as said aliens [[ScrewThisImOuttaHere had apparently left the Imperials to their deaths.]] This is eventually subverted when [[spoiler:the Eldar]] join in alongside the Imperials and Knights.
* EverybodysDeadDave: Apart from the Emperor, the only other characters left alive from the events of the anime are CC, Nunnally, Suzaku and Kallen. [[spoiler:Who also happen to be the immortal leaders of the Black Knights. Euphie has also been revealed to be alive somehow.]]
* ForegoneConclusion: The Fall [[spoiler:and subsequent Exterminatus]] of Ultramar at the beginning is meant to show just how desperate the Imperium's become. And how close Abaddon is to Terra itself. Likewise, all the flashbacks involving Lelouch's plans being set into motion prior to becoming Emperor.
* FutureImperfect: An appendix reveals that many of the Space Marines' features and abilities were reverse-engineered from fragments of data relating to the [[spoiler:Black Knights']] creation, though some fragments were less complete than others. The Astartes' [[ImAHumanitarian ability to absorb knowledge by consuming flesh]] was taken from a heavily corrupted science log...which conveniently left out the bit on the ''mere notion'' as an affront to nature. The Black Knights themselves actively try to avert this, which is reflected in their names, designations and traditions, as a result knowing more about the past than the Imperials.
** The events of the anime itself are long shrouded in myth, if not outright forgotten by the 51st Millennium. Although the Adeptus Mechanicus and High Lords of Terra have at least a dim idea of what happened in the distant past. [[spoiler:Such as the Black Knights' bunker in Mars, CC's identity and the old tales of Zero.]]
* GambitRoulette: One example involves CC leaving behind an STC containing codes that would allow the Black Knights to communicate with the Imperium should they return, certain that no one would bother to ''change'' the frequencies in that time.
* GodzillaThreshold: Even if the Imperium at large was unaware of it at first, the Black Knights effectively serve as this, which is at least partly Lelouch's plan all along.
* HiddenElfVillage: The Black Knights, with all their technology and skill, only number in the hundreds of millions. And even with its severely reduced size, the Imperium ''still'' has more than enough people, guns and ships to dwarf them. The Imperial Navy's Lord Admiral even lampshades the fact that they wouldn't fill up the Administorum alone.
* HowWeGotHere: There are flashbacks stretching to the events just after [[CodeGeass Zero Requiem]] that slowly explain how things got the way they are in the 51st Millennium. As well as why CC and the others are still alive.
* ALighterShadeOfGrey: The Knights, compared to the Imperium, are well-meaning and considerably less grimdark in general. Though that said, their actions, motives and general seclusion aren't exactly altruistic. This is even more evident with their immortal leaders.
* LukeYouAreMyFather: Lelouch created the Primarchs from tissue samples taken from [[spoiler:Kallen and CC. Kallen learns this from her 'son', Fulgrim.]]
* TheMasquerade: How the Inquisition attempts to deal with the Knights initially. Sightings and reports are passed off as either STC discoveries or Tau/Eldar/etc. attacks.
** It breaks entirely when CC and Nunnally decide to pay the Inquisition a visit...and subsequently manage to put it ''and'' the Grey Knights in their place by simply talking.
* MedievalStasis: Though more scrapped for manpower and resources, the Imperium is still virtually identical to its 41st Millennium self. The Black Knights also show signs of this trope, though to a lesser degree. Compared to the Imperials, they're more or less undergoing ModernStasis, with many of its advancements carried over from the Dark Age of Technology.
* MiniMecha: The Knightmare Frames of the 51st Millennium have evolved from their ancient predecessors to be something like the Tau Battlesuits, although they've retained some of the appearances and functions of their distant namesakes. They're also coincidentally about the size of a fully-armed Space Marine.
* MythologyGag: To both source materials.
** The "World of C" for instance is shown as a stable pocket within the Warp.
** It's mentioned in passing that someone who might have once been [[spoiler:Milly Ashford]] had some Warp-fueled adventures in Commorragh involving some hapless Dark Eldar.
** The "present" date is stated to be 017.M51...which would make it ''precisely'' 48,000 years after the events of the anime, assuming that the Imperial Calendar is modeled on the a.t.b. one.
* OurGhostsAreDifferent: The Warp being known as the Guardian is revealed to be this. Especially since she used to be [[spoiler:Euphemia]].
* PsychicPowers: While not quite to the same level as the Emperor, it's mentioned that both [[spoiler:CC and Nunnally]] could destroy whole armies with their minds alone. Though given that they've had nearly 50,000 years of experience, it was bound to be expected. The powers themselves are hinted as having evolved from Geass.
* TheRemnant: The Black Knights are themselves a relic of sorts from the Dark Age of Technology as well as Lelouch/the Emperor's original vision of the Imperium.
* TimeAbyss: The Black Knights' self-imposed exile in the Warp is implied to be this (over ''20,000 years'' in hiding). Their immortal leaders on the other hand are even more so, recalling the events of the anime and the ''original'' Knights under Zero.
* TookALevelInBadass: On the 40K side of things, Abaddon's Black Crusade has not only ''succeeded'' but is by the start of the fic on a direct course to Terra itself. And then you've got the Black Knights themselves [[spoiler:particularly CC, Nunnally and a few others]], who have thousands of years' worth of experience by this point.
* UsedFuture: Considerably more so than before. Many of the Imperial Navy's more "recent" ships for instance look much more ramshackle and hastily assembled than the ones fielded in the 41st Millennium.
* VestigialEmpire: The Imperium by the 51st Millennium has largely been reduced to Segmentum Solar and scattered remnants of its once vast territories. Manpower and resources are increasingly become a luxury, the dwindling armies of Guardsmen and Space Marines have fallen back ever closer to Terra and the High Lords are for once uncertain about their dominions' very survival. [[spoiler:And with Lelouch/the Emperor disconnected from the Golden Throne in an attempt to save his life, it's all but collapsed beyond the solar system]].
* WeHaveReserves: Despite being literally on its last legs, the Imperium ''still'' has ample supplies of manpower and resources to throw against the forces of Chaos. This time however, the Imperials are starting to find even ''those'' to be increasingly precious and limited. The Black Knights meanwhile avert it in comparison, given that they place high value in avoiding casualties despite numbering in the hundreds of millions.