While X-Men: Apocalypse received a mixed reaction from fans and critics alike, one scene got fans buzzing. The post-credits sequence teased the arrive of Mister Sinister, one of the biggest villains in X-Men history. Fans were expecting Sinister to show up in Logan and even assumed when Richard E. Grant was added to the film's cast that he would be playing the villain.
All of that speculation turned out to be wrong. Mister Sinister did not show his pale face in Logan, but there's a hint in the film that he may have been looking in the background and around the edges.
If you missed the tag for X-Men: Apocalypse, it involved a man entering the Weapon X facility where Jean Grey, Cyclops, and Nightcrawler had earlier freed Wolverine, and then Wolverine subsequently went on a rampage, killed most of the facility's staff, and ran off into the woods.
The person in question entered the facility to retrieve blood mutant DNA samples from the facility. The camera revealed that the case the DNA was being placed into was the property of Essex Corp. Mister Sinister's real name is Nathaniel Essex, hence the connection to the X-Men villain.
Naturally, fans believed that Grant's character would turn out to be Nathaniel Essex and that the corporation that created X-23 in Logan would be Essex Corp. That turned out not to be the case. Grant played Zander Rice, the scientist who helped create X-23 in the Marvel Comics universe. Rice's company was not Essex Corp, but Alkali Transigen.
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However, there may still be a link Alkali seems to be a reference to Alkali Lake, which was the region in Alberta, Canada where Weapon X headquarters was located. Rice reveals that his father was one of the scientists stationed there when Wolverine escaped.
So Wolverine was held and experimented on at Alkali Lake, where mutant DNA was stored. That DNA was taken into the custody of Essex Corp. One assumes that Essex Corp must have ties to or some kind of arrangement with Weapon X, which was a government run project.
So then how does Alkali Transigen come to possess the DNA needed to create its mutants? Rice is connected to Weapon X through his father and Alkali Transigen seems to be named after the old Weapon X facility.
At some point, it would seem that, at some point, Essex Corp gave the DNA over to Alkali Transigen, something that producer Simon Kinberg confirmed previously. However, there are many years between X-Men: Apocalypse and Logan, so it seems like that Sinister would have done some work on the DNA and the mutant creation process on his own time, even before Rice was old enough to complete the work. We can even speculate that Alkali Transigen could be a subsidiary of Essex Corp, or a rebranding of Essex Corp.
This is all interesting because it opens up a lot of potential for future X-Men movies. It seems unlikely that the X-Men movies would so specifically tease Mister Sinister and then not have him appear in a film, which means he's probably planned to have a role in New Mutants or whatever the next core X-Men movie turns out to be.
That Sinister had access to the Wolverine's DNA during the years after X-Men: Apocalypse is important because it provides a way to introduce X-23 or at least a version of X-23, that doesn't involve time travel. Sinister could theoretically clone X-23 from Logan during the years between X-Men: Apocalypse and Logan, as different X-23 that could then join the cast of young X-Men being featured in the core films.
Mr. Sinister's involvement could even provide a connection between the core X-Men movies and the Deadpool and Gambit spinoffs. Gambit worked for a Sinister for a time before the events of the "Mutant Massacre," which could be adapted in some way for the Gambit movie. In Deadpool, Ajax and Angel Dust are working for the Weapon X program, which we already know has some connection with Essex Corp.
This is a lot of speculation, but there's a lot of potential here. Now fans just need to see how Fox decides to use it.