As far as any credible research into the subject is concerned, The Illuminati does not exist. It did, however, exist at one point, as a "secret society" with a relatively small membership founded as an adjacent-alternative to The Freemasons in Upper Bavaria in 1776, which was suppressed out of existence in 1785 and never reconstituted in any meaningful way.
The name "Illuminati," however, became used as a shorthand by the ruling class for various anti-monarchy revolutionary movements in Europe throughout the 1800s, with some claiming (absent any evidence) that the group itself had even covertly endured. This shorthand was later revived and become popular among conspiracy theorists, and the idea caught on in popular culture, which has since remained fixated on the name (helpfully, defunct 18th Century social-club handles are public-domain).
Today everyone - from hip-hop fans suspicious of Jay-Z's business success to professional conspiracy theorist David Icke, who preaches that certain world leaders are secretly a race of shape-shifting lizard people - uses "Illuminati" as either a literal or figurative identifier for a supposed "they" pulling the world's strings from behind the scenes, and works of fiction continue to reflect the concept's popularity. Villains calling themselves "Illuminati" have turned up everywhere from The DaVinci Code books to The Cleveland Show to Disney's Gargoyles.
So, let's talk about what "The Illuminati" means in the world of Marvel Comics - and might it mean for the future of the Marvel Cinematic Universe.
BEST LAID PLANS
Like a lot of "big idea" concepts in early-2000s comic book writing, Marvel's Illuminati started out as an attempt to both recontextualize popular stories from the past and explain away well-known continuity problems. It bears a certain level of similarity to the revelations of the DC miniseries Identity Crisis, which was premised on the idea that a subset of the Silver Age Justice League had covertly used mind-altering powers to "handicap" the abilities, memories and behavior of certain supervillains - explaining (in part) why DC bad guy schemes of that era had often taken cartoonish, less-than-sinister form. Marvel's "Illuminati" storyline, however, was aiming at a bigger idea: What if a group of powerful, well-connected characters (i.e. superheroes who were also team leaders, politicians, businessmen, etc) were playing a secret behind-the-scenes role in the history of some of Marvel's pivotal events?
First revealed by writer Brian Michael Bendis in the pages of New Avengers in 2005, Marvel's Illuminati have no association with the historic one save for borrowing the name. The group's history is related in the New Avengers: Illuminati miniseries via flashbacks set during classic storylines from the previous fifty years (or, in Marvel Time, the last decade) of Marvel continuity. They form during a meeting called in secret by Tony Stark and attended by Prince Namor (The Sub-Mariner), King T'Challa (Black Panther), Doctor Strange, Professor Charles Xavier, Reed Richards and Black Bolt shortly after the events of The Kree-Skrull War, which saw The Avengers fighting to prevent Earth from being caught in the crossfire of an intergalactic war between the two alien races.
Stark, having reasoned that the situation could have been better resolved had disparate heroes and teams shared their individual experiences with the Kree and Skrulls prior to the War, proposes that this group - together representing the collective leadership of established Marvel superheroes - agree to meet periodically, compare notes and help each other out in secret. Essentially, it's a ready-made explanation for why heroes from different factions and corners of the universe seem to run into one another in the right place at the right time. Initially, the only member who refuses to join is T'Challa, who warns that trying to manage the world in secret is going to blow up in their faces (spoiler: it does).
The story (the original, at least, though "The Illuminati did it!" remains a favorite mythology-shortcut to this day) then tracks The Illuminati's actions as the "real story" behind major events like Secret Wars and the backstory of The Runaways, answering loose-thread questions from Marvel history and patching continuity holes. It also lays seeds for later stories, the most prominent being an attempt by The Illuminati to collect and destroy The Infinity Stones; which doesn't work and leads to them being divided among the membership to be protected/hidden (this also does not work).
Oh, incidentally: Black Bolt has actually been a Skrull imposter almost this entire time, as have multiple associates of each member's respective team/group/family. That's going to be important later. In any case, the Illuminati "history" comes to an end in the then-present, as part of the setup and lead-in for the Civil War event in 2006.
PLANET HULK & CIVIL WAR
What is at first said to be the final "proper" meeting of the Illuminati (sans Professor X - long story) serves as the foundation for the year-long "maxiseries" Planet Hulk, in which the group meets to finally settle the problem of The Incredible Hulk, whose most recent out-of-control rampage has leveled most of Las Vegas. Tony Stark proposes an unthinkable last-ditch solution: Trap Bruce Banner in a space probe and shoot The Hulk off to a distant, uninhabited planet where he can live but no longer endanger anybody. Namor dissents and quits the group, but the others reluctantly agree that it's the only option they have left, and the plan goes into action.
Granted, most of this angle seems to exist out of necessity (Civil War would have ended pretty quickly had either team had The Hulk on their side, so he needed to be somewhere else for a year) but regardless Planet Hulk, which ran through a year's worth of the monthly Incredible Hulk books, turned out to be one of the best-received Marvel series in a long time. In the story, the ship carrying The Hulk crashes on the wrong planet, which turns out to be habitable and populated by a diverse empire of aliens among whom Hulk's appearance isn't at all unusual and his strength/ferocity are viewed as a (literal) gift from above. He becomes a gladiator, a revolutionary leader, assembles a loyal group of warrior allies and finds a lover in the planet's rightful queen. Unfortunately, happiness is short lived, and soon enough Hulk is on his way back to Earth with an entire intergalactic armada at his command, seeking revenge upon the Illuminati.
Back on Earth, the group meets one more time as Tony Stark attempts to convince the remaining members to support and get behind the nascent Superhero Registration Act (the comics' version of The Sokovia Accords) before the issue is forced by some bad turn of events that he believes is inevitable. Namor thinks it's none of Atlantis' business, Professor X is otherwise occupied with other Mutant problems (this was the point where the push to separate The X-Men from the rest of Marvel was really getting going), Doctor Strange and Black Bolt say no, and that's the for-real-this-time end of The Illuminati - leaving Tony Stark and Reed Richards as the most prominent supports of Registration when things do, in fact, go bad shortly thereafter.
Civil War, of course, plays out as most already know: Anti-Registration heroes lose, Captain America surrenders and gets assassinated (he gets better), and Iron Man technically wins but at the cost of basically everything he loves. The Illuminati don't formally reunite, and are instead pushed back together by force in the next big storyline...
WORLD WAR HULK & SECRET INVASION
On a meta level, the entire Illuminati concept is essentially about how imposing a unified order on the comings and goings of Marvel superheroes just isn't a great idea, and the big post-Civil War events are largely about the participants thereof paying for that hubris.
Firstly, in World War Hulk. When The Hulk's alien army shows up and effectively conquers Earth, he's got a very specific plan in mind: Forcing the members of The Illuminati to fight one another in gladiatorial combat in Madison Square Garden. He manages to make his point while still showing final mercy, but things end on a muddier note after a final twist absolves The Illuminati of at least one of the main details Hulk was most upset about, and Iron Man turns out to have one last contingency Hulk plan to put into effect. Overall, the story has the effect of establishing that the Marvel heroes are even more disorganized post-Civil War than they'd thought - which is bad news, as the next shoe to drop involves what The Illuminati had thought was their main successful venture.
In Secret Invasion, it turns out that The Skrulls reneged on their detente with The Illuminati and have been quietly seeding Earth with shape-shifting sleeper agents since the end of the Kree-Skrull War. With everything suitably destablilized, said sleepers are activated and humanity wakes up to realize they've already been successfully invaded and didn't even realize it. The good guys manage to get their act together (hobbled further by a bunch of major characters turning out to have been Skrull imposters) and just barely enough to eke out a win, but because he's the one to pull the trigger and put down the Skrull Queen during the big final melee, the public credit all ends up going to... Norman Osborn (it's complicated), with the final stinger providing a nasty closure to the initial Illuminati angle as the onetime Green Goblin sets up his own secret brain-trust of prominent villains (Doctor Doom, The Hood, Loki, The White Queen and Namor), called "The Cabal."
SECRET WARS & BEYOND
The original Illuminati never fully reunite as an "official" regularly-meeting group, but they do re-team informally as part of several subsequent storylines after largely sitting out (for a variety of reasons) the Dark Reign, Fear Itself and Seige storylines (along with more "contained" events happening in the X-Men, Spider-Man and cosmic sides of the Universe). They manage to prevent The Hood from acquiring The Infinity Stones and once again set about hiding them, which turns out to be the last mostly-successful act they undertake.
Otherwise, the group (after failing to assemble during the events of Avengers Vs. X-Men) once again becomes involved in the sprawling, years-long storyline conceived by Johnathan Hickman during his epic stewardship of The Avengers and associated titles, which involves the Marvel Heroes facing an unimaginable threat (various alternate-realities in the Marvel Multiverse are colliding with each other, causing inter-universal devastation) and a reconstituted Illuminati being a key part of the unthinkable solution: Destroying other worlds in order to save their own.
There's a lot of twists involved in this (still fairly recent) storyline as to who does what and why, but suffice it to say the question of who is versus who isn't willing to do this proves the ultimate undoing (well, for now) of the Illuminati. They individually have hands in setting up the temporary new reality created during the second Secret Wars event (i.e. "Battleworld,") but in the subsequent new reality spinning out of that culmination they aren't really a thing anymore. Instead, an entirely new "evil" version of The Illuminati is making trouble in the new Marvel Universe under leadership by The Hood.
But what does any of this mean for the Cinematic Universe and Doctor Strange specifically - and why are we talking about it now?
Thus far, the idea of the Illuminati showing up in The Marvel Cinematic Universe hasn't come up much even among hardcore fans - largely because the only major players to have shown up in the movies thus far have been Iron Man and Black Panther, the latter's role thus far being confined to a supporting part in Captain America: Civil War. Professor X is currently unavailable given the X-Men living exclusively in the Fox-owned Mutant Universe, Namor has no movie in development, Black Bolt's franchise is on indefinite hold (though still playing a side-story role in the Agents of S.H.I.E.L.D. mythos), and while The Winter Soldier established that Doctor Strange already exists somewhere, he's not due to appear in full for another month.
In fact, the very reason the Illuminati has come up is because Strange actor Benedict Cumberbatch brought it up himself. During a recent set visit, the famously talkative Sherlock star offered the following about his foreknowledge (or lack thereof) of future MCU events:
"I think playing any iconic role when you’re stepping into big shoes, into the shadow of people who have come before you and you can’t process that in a movie to movie basis …I’m excited to see where the Illuminati and whatever else might happen, how that works, and where it ends up. So, yeah, I’m aware of his place within the comic pantheon of it all, the Marvelverse, but I don’t email Kevin [Feige] saying, “When are we doing next film?” I’m excited to see. I’m excited to see. And as you know, from all these previous incarnations, they play out in unexpected ways from the comic format and journey, so they manage to both fulfill that magical space of doing things that seem to please diehard fans and bring something new as well. So, I guess that’ll be the centerpiece for this guy’s journey."
What does any of that actually mean, outside of "he doesn't know?" Hard to say, but it feels like the first time the word "Illuminati" has come up from anyone officially connected to the Marvel Universe production-machine. Does this mean that the Illuminati - or a version of them - are due to take shape in the coming years of the MCU? Maybe - though for the group to exist in anything close to its familiar form or function from the comics feels rather unlikely at this stage. Even setting aside the fact that replacement-players would need to be found for Reed Richards, Professor Xavier, Black Bolt and Namor, there's the question of exactly what the point of having the group exist would be in terms of storyline necessity.
After all, the entire "gimmick" to the Illuminati of the comics was to reveal hidden connections and machinations behind previously-told stories; and while it's theoretically possible that future Marvel films could reveal that a group of good guys were secretly nudging events into proper alignment all along, it's difficult to discern what the point of that would be. As is, the Cinematic Universe has already been largely defined by events being manipulated behind the scenes: S.H.I.E.L.D. was largely running things up through the first Avengers movie, and Winter Soldier revealed HYDRA as a puppet-master within the puppet-master. As of Civil War, the U.N. is in charge of superhero comings-and-goings. Daredevil has Stick and the Hand both shoving his destiny in various directions. Two and a half entire seasons of Agents of S.H.I.E.L.D. were premised on the idea of a massive intelligence agency operating entirely behind the scenes. What, exactly, would the Illuminati be adding to those equations?
It's possible that they could arrive as a plot point for the buildup to Infinity War (or whatever they end up calling it) as a shorthand for how characters like the now-scattered Avengers, Guardians of The Galaxy and whoever else turns up get on the same page for fighting Thanos. If so, it still feels (somewhat) less than likely that "Iron Man and several others were actually part of a whole other secret team all along!" would be the form it takes - perhaps more likely, a Cinematic Universe Illuminati might involve supporting characters with ambiguous backgrounds (Martin Freeman's oddly conspicuous government functionary from Civil War? Peter MacNicol's undercover Asgardian from Agents of S.H.I.E.L.D? Stick? Where's Nick Fury been, anyway?) or perhaps incoming characters we haven't met yet - it certainly would be a novel way to get Ms. Marvel into the picture.
Or maybe Cumberbatch was talking about something else entirely. As outlined previously, the word "Illuminati" is more widely-known to popular culture than the majority of Marvel story details, so the movies could be breaking out the term for an entirely separate purpose: A new group of villains, perhaps, closer to the popular conception of The Illuminati, to fill in for the largely-defunct HYDRA? It's entirely possible that we may not know until Doctor Strange hits theaters on November 4th - though fans should at least find it extremely encouraging that it's still possible for a real mystery to exist even after almost a decade of MCU continuity.
Doctor Strange opens November 4, 2016; Guardians of the Galaxy Vol. 2 – May 5, 2017; Spider-Man: Homecoming– July 7, 2017; Thor: Ragnarok – November 3, 2017; Black Panther – February 16, 2018; Avengers: Infinity War – May 4, 2018; Ant-Man and the Wasp – July 6, 2018; Captain Marvel– March 8, 2019; Untitled Avengers – May 3, 2019; and as-yet untitled Marvel movies on July 12, 2019, and on May 1, July 10, and November 6 in 2020.