At the end of "X-Men: Apocalypse," we see a briefcase emblazoned with an "Essex Corp." logo that contains a sample of Wolverine's DNA. That, of course, is a reference to Mister Sinister, a major X-Men villain from the comics who may at last make his film debut.
So what's Mister Sinister's deal, exactly?
He's first mentioned in 1986's "The Uncanny X-Men" #212, during the Mutant Massacre, when Sabretooth refers to a "Mr. Sinister" who sent the assassins called the Marauders to kill the subterranean Morlocks. It wasn't until "The Uncanny X-Men" #221 readers that actually got to see Mister Sinister as he berates his Marauders for failing him.
You might think his name and his appearance are oddly over-the-top -- and that was intentional. You see, writer Chris Claremont originally intended for Mister Sinister to be the name that a powerful boy came up as the psychic manifestation of his abilities. It's supposed to be hokey, as it was thought up by a kid who never ages, and ends up using Sinister more and more as his public persona. In the backup stories in 1989's "Classic X-Men" #41, we see Scott Summers from his time an orphanage and his fellow orphan Nate, who's obsessed with Scott. In Claremont's original plans, "Nate" was Sinister, but that was later retconned so that the boy was merely Sinister in disguise.
Sinister's obsession was revealed to have gone even further during the 1989 crossover "Inferno," where we learned in "X-Factor" #39 that he had cloned Jean Grey to create Madelyne Pryor so that Scott would procreate with her. Sinister had even come up with the ruby quartz glasses needed to restrain Cyclops' eye blasts! Cyclops seemingly destroyed Sinister at the end of "Inferno," but he soon popped up again.
During the 1992 "X-Cutioner's Song" crossover - specifically "X-Men" #14 -- Sinister captured Jean Grey and Cyclops and delivered them to Stryfe, a clone of the son of Scott Summers and Madelyne Pryor, in exchange for Stryfe's genetic material. Basically, try to think of Sinister as Lucky the Leprechaun and Summers DNA as his Lucky Charms. Sadly it was revealed in "X-Force #18" that Sinister wasn't given Summers genetic material but instead received a vial containing the Legacy Virus, which was now unleashed upon the world.
A few years after "X-Cutioner's Song," we finally learned Sinister's official origin (as opposed to Claremont's intended origin) during "The Further Adventures of Cyclops and Phoenix." In the story, Scott Summers and Jean Grey end up in mid-19th England, where a man named Nathaniel Essex and his wife Rebecca meet En Sabah Nur (aka Apocalypse). Essex is obsessed with genetics, but he and his wife are strained by the loss of their son. En Sabah Nur offers to mutate Nathanial Essex, who initially refuses. However, the loss of his wife (and her hatred of him at the end) drives him to accept, and Apocalypse's experiment leads to the creation of a new being known as ... Sinister!
Sinister kept popping over the next couple of decades, particularly in connection with Gambit, with whom it was revealed he shared a history. In fact, Gambit assembled the Marauders for Sinister, and even led them to the Morlock tunnels. However, Gambit who was paid with a mysterious vial that was never fully explained, hadn't realized the Marauders were about to slaughter the mutants. So Sinister and Gambit have crossed paths a number of times (as we've seen, once Sinister has taken an interest in you, he will not let it go). Sinister was involved with a newly revived Weapon X program, which is when we learned he had worked with the Nazis during World War II because they allowed him to conduct all sorts of messed-up experiments.
Sinister's next major storyline was the 2007 "X-Men: Messiah Complex" crossover. After "House of M," Scarlet Witch had erased most of the mutants from Earth, so when a mutant girl was born, everyone was interested in finding her, including, of course, Sinister, who planned to use for experiments. He had Mystique form a new version of the Marauders, and even enlisted Gambit to betray the X-Men to help find the baby (Sinister never forgets, Gambit!). At the end of the storyline, Sinister was seemingly killed.
It turns out, however, that someone like Sinister is prepared for virtually anything. In this instance, he ended up in a cloned female body, launching a brief period (beginning in 2008's "X-Men: Legacy" #214) in which the character was Miss Sinister. Of course, that change was only temporary, and eventually Mister Sinister returned.
Mister Sinister was a particular thorn in the side of the X-Men during their time in San Francisco and Utopia, as he merged with the Dreaming Celestial to become super-powerful. Even after his defeat, he popped up again in the area, having built a Victorian-inspired subterranean city. Sinister played a major role in the 2012 crossover "Avengers vs. X-Men" when the "Phoenix Five" took him on and discover he had created a clone army. They destroyed seemingly every clone, but Sinister managed to survive yet again.
Most recently, Sinister showed up in the pages of "Extraordinary X-Men," where he spliced together mutant and Inhuman DNA in an attempt to prevent Terrigen Mists from killing mutants. That was when we learned that Sinister experiments on clones of various X-Men, including -- of course! -- Cyclops.
It certainly won't be surprising if we see Sinister conduct experiments in the next Wolverine film.