Note: The following article contains spoilers for X-Men: Apocalypse. Duh. 

A new X-Men movie is in theaters, which means there’s another post-credits sequence to dissect. Director Bryan Singer made his triumphant return to the franchise with the 70s-set X-Men: Days of Future Past, which very clearly set up the following installment with its post-credits sequence teasing the villain Apocalypse in Ancient Egypt. Indeed, the world’s first mutant took center stage in X-Men: Apocalypse, which is now playing everywhere, but what did Singer and Co. decide on to follow this film up? Well, it’s a bit less clear.

If you’ve seen X-Men: Apocalypse and you stayed until after the credits, you saw a sequence set in the aftermath of Wolverine/Weapon X’s murderous rage at Stryker’s Alkali Lake facility, where Stryker was jailing (and experimenting on) mutants. In the post-credits scene, we track a handful of Men in Black-looking mysterious folks, who walk over the many, many dead bodies to head straight to a lab facility where they collect what appears to be a blood sample from Weapon X. This sample is added to a briefcase that houses a number of other different colored samples, and the stinger comes when the briefcase closes and we see the name “Essex Corp.”

So, yeah, you may be a little confused. First, Weapon X. In the comics, the Weapon X program is a secret government project conducted by William Stryker, Jr. in which he turns mutants and humans into living weapons via enhanced superpowers. Wolverine marked the tenth version, and was therefore referred to as Weapon X. So in this case, Weapon X is Wolverine who has been “enhanced” by Stryker, which one assumes is where he gets his Adamantium Claws in this post-Days of Future Past timeline. The main point of this post-credits scene, though, is the “Essex Corp.” name, which appears to be teasing a villain known as Mr. Sinister. The character was born Nathaniel Essex and became a biologist in 19th Century London, where he developed a strong fondness for Charles Darwin’s theories of evolution. Essex is a bit of a eugenics-influenced baddie, as he became convinced that Darwin’s theory was shackled by too many moral constraints and set out to perfect human evolution by emphasizing what he called “Essex Factors” in the human genome, which he felt were the cause of increasing mutation. Following the death of his son, Essex began a downward spiral that saw him mocked by his contemporaries for his unorthodox ideas and experiments. After being confronted by the awakened Apocalypse, who supported Essex’s ideas, his experiments grew even more grotesque, going so far as to dig up his deceased son to perform further work. Upon realizing the extent of his experiments, Essex’s wife grew disgusted, and with her dying words described him as “utterly… and contemptibly… sinister.”

With his child and now wife dead, Essex went full villain, and Apocalypse transformed him into an ageless being with telekinesis, at which time he took on the name of “Mr. Sinister.” From there, the comics stories grow more complicated and convoluted, so it’s unclear how, exactly, Mr. Sinister will surface in the X-Men films (especially since Apocalypse won't seem to factor into his origins), but the intention with this post-credits scene certainly seems to be to tease his arrival.

Given Sinister's obsession with genetics and perfecting the genetic code, sothe collection of blood samples here via these Men in Black seems to suggest that Weapon X’s blood may be put to villainous use in Sinister’s pursuits. But the question then becomes, which movie is this post-credits scene teasing? Singer and writer/producer Simon Kinberg have revealed their intention to set the next X-Men sequel in the 1990s, but there are a lot of behind-the-scenes factors that could inform how soon we might see another proper X-Men sequel. Contracts were up for Jennifer Lawrence, Michael Fassbender, and James McAvoy at the end of Apocalypse, and per their new contracts, if one of them decides not to return in the future, none of them will. That complicates matters as these are three incredibly busy performers, but it’s possible we could see the 90s-set X-Men film only involve McAvoy’s Professor X in a less prominent role—like a glorified cameo, perhaps?

The X-Men universe continues to expand, however, so it’s also possible that Mr. Sinister pops up in a different film altogether. Production is underway on Hugh Jackman's final Wolverine movie, The Fault in Our Stars filmmaker Josh Boone is writing and directing New Mutants, which could start filming early next year, and Deadpool 2 is deep in development and on its way to building to an X-Force movie. And then there’s Gambit, which may or may not eventually happen. So there are a lot of different ways Mr. Sinister could pop up and wreak havoc. Singer is next directing a new iteration of 20,000 Leagues Under the Sea, so if he's directing the next X-Men movie, the turnaround here won’t be as quick as Days of Future Past and Apocalypse. We may have a while to wait, but Mr. Sinister’s arrival is definitely imminent.