We don't know who the villain of X-Men: Dark Phoenix is, but based on the characters involved and other inspirations, there's a chance it could be bizarre 1990s foe Onslaught. Time was, 20th Century Fox's X-Men movies were notorious for still conducting their superhero business the 90s way: while Sam Raimi's Spider-Man movies made fidelity to the more whimsical visuals of the comics "cool" again in 2002, the Mutant heroes stuck with their black rubber body-stockings until well into the early 21st Century. When the Marvel Cinematic Universe turned building whole franchises around famous superhero storylines straight from the four-color source, Wolverine and company were long content to merely nod in the direction of their more esoteric comic-book adventures - with even the (originally) galaxy-spanning Dark Phoenix Saga being reduced to a pair of lackluster earthbound brawls in the woods and a prison yard.That all changed with Matthew Vaughn's continuity-rewriting prequel X-Men: First Class, which dropped a more classically youth-oriented team into a candy-colored retro adventure (complete with the old-time "yellow spandex" uniforms the first film had openly mocked) featuring a clash with The Hellfire Club. The success of that film was followed by sequels that openly embraced the most "out-there" elements of the property, like time travel (Days of Future Past) and ancient god-like nemeses (Apocalypse), along with The Wolverine and Logan allowing Hugh Jackman to finally portray the most true-to-source version of his star-making role possible.Related: How X-Men: Dark Phoenix Will Improve on ApocalypseNow, with a deal that could complete this transformation by reunifying the X-Men with the rest of the Marvel Cinematic Universe apparently in the offing, the upcoming Dark Phoenix feature aims to keep the trend going with a "do-over" adaptation of possibly the most famous (and famously bungled by the films) X-Men story of all.The Real Villain of Dark Phoenix Is A Mystery (This Page)But while fans have thrilled to the news that Dark Phoenix will indeed feature a trip to outer space and big-scale alien action setpieces (both hallmarks of the original comics storyline, which saw Jean Grey transformed into an unstable super-powered alternate-persona after a battle in outer space), the production has been largely mum on what the rest of the plot will involve - including whether or not there will be an additional villain separate from Phoenix herself. Many believe this to be Jessica Chastain's yet-unnamed blonde character recently teased by EW; a mystery player who some believe will be a leader of The Skrulls, a shape-changing alien race who are uniquely available (with some caveats) to turn up in both the X-Men and Marvel Cinematic Universe continuities.But the same Entertainment Weekly preview of the film that has now cryptically confirmed that the film will feature a "massive twist" at its midpoint that will "change the direction of the franchise" may also contain the first clues to what the twist itself could be - with many fans zeroing in on key details looking for subtle hints or "easter eggs." The word "sinister" pops up in a quote, possibly related to a famously bizarre X-Men nemesis called Mr. Sinister whose presence was teased in post-credit scenes from Apocalypse and may have been connected to the possible-future events of Logan. But the report also makes a point to describe the X-Men as being put in danger by the actions of their leader, Charles Xavier... specifically "his growing ego."On one level, that could be a fairly benign statement: if Professor X is experiencing a touch of egomania, his decisions could be framed as causing whatever disaster will lead to Jean Grey's transformation (in the original X-Men comics, "Phoenix" was originally presented as an extreme evolution of her original Mutant powers but later revealed to be the manifestation of her possession by a godlike cosmic entity - thus far the film versions have focused only on the first part). And it would jibe with the story's inciting incident apparently being the X-Men getting hit by a "solar flare" while undertaking a highly-publicized rescue mission in space. But for a certain stripe of longtime X-Men fanatics, any talk of trouble coming from the dark side Professor Xavier's powerful mind is going to call to mind a single word: "ONSLAUGHT.""Onslaught" is technically both a name and a title, the second referring to a massive crossover event that centered on the X-Men books but spread over and reverberated throughout a wide swath of the Marvel Universe in the 1990s. It's infamously remembered as one of the biggest selling events of its kind at the time but also with disdain - regarded by many as an example of everything that was going wrong with Marvel event comics (and X-books in particular) at the time and tarnished further by association with its successor-event, "Heroes Reborn." But as a name, Onslaught refers to one of the X-Men's most powerful villains: a giant armored behemoth that was actually a "sentient psionic entity" created through the attempted fusion of Professor X and Magneto's pysches... basically, Onslaught is what happens when Charles and Eric make a Brain-Baby.Once materialized, Onslaught does what Marvel villains of nebulously-defined godlike power tend to do and attempts to conquer the world and remake reality in his own image - and is so effective at doing so that he can only be thwarted by the combined forces of the X-Men, Avengers, Fantastic Four and even Reed and Sue Richard's ultra-powerful Mutant psychic son Franklin. The world is saved, but at the heavy cost of the Avengers and Fantastic Four being killed off in battle... or, rather, seeming to be killed off: that ending actually served as an excuse to temporarily zap the "dead" characters into Rob Liefeld and Jim Lee's widely-despised alternate-universe "Heroes Reborn" event.Related: The Worst X-Men Villains Of All TimeAs you'll gather, the Onslaught of the comics is a bit on the silly side even as a world-killer X-Men punching-bags go. But the basic idea (i.e. Xavier + Magneto = Invincible Monster Foe) has always felt like something the reflexively Charles/Eric-centric movies might try and do a less preposterous spin on. No, a giant-sized Anime-inflected version of Magneto is unlikely to be stomping across the screen, but McAvoy, Michael Fassbender, Patrick Stewart, Ian McKellen or some combination thereof showing up with new threads and a different attitude as a movie-verse version of Onslaught is easy to imagine - and, notably, McAvoy has evidently not been asked to drop the bulked-up physique he crafted for M. Night Shyamalan's forthcoming Glass.On the other hand, even if Onslaught feels like an idea the X-Men movies will want to take a swing at eventually, it's hard to see him playing a central role that might take away focus from the long-awaited "proper" Dark Phoenix movie. Perhaps it's more likely that, if he did turn up for this particular adventure, it would be in the context of a background threat to be dealt with later - or something to be revealed in a post-credits scene. Perhaps his "origin" would come as a necessary risk - a desperate big to create a power that could counter that of Phoenix, only to now have a new heavy waiting for the next film.Or - perhaps even more tantalizingly to fans - Onslaught's association with re-wiring the status quo of reality could turn out to be one of the answers to loose plot-threads or continuity-hiccups to come in need of resolution as the X-franchise prepares for a potentially-upended future. One thing is for certain: in this new age of the X-Men franchise, even absurd nonsense like Onslaught may no longer be off-limits to filmmakers at Fox.Next: Why I'm Worried About X-Men: Dark Phoenix