Sinister, the brilliant but amoral geneticist Nathaniel Essex, was first mentioned in relation to the Gambit movie when rumored details about the plot surface in September:
"We jump to 10 years later; Gambit is hired to do a job in Paris to steal something from the Louvre Museum. It was apparently a test job to see if he still has it. His mysterious employer is revealed to be Nathaniel Essex a.k.a. Mr. Sinister. He offers Gambit 40 million to recover a mysterious trunk that was stolen by the Boudreaux clan. It will be auctioned off during the yearly Thieves Ball where all the criminal organizations in the world meet up. It uses New Orleans' Mardi Gras celebrations as a cover. Gambit decides to enlist a crew of mutants to pull off this seemingly impossible heist…"
For those who don't know who Mr. Sinister is, he is arguably the X-Men's greatest foe next to Magneto and maybe Apocalypse. While not a mutant, Sinister has been genetically altered and has shown a host of different superpowers over the years, including regeneration and psychic abilities.
Nathaniel Essex was born in Victorian-era London but has lived an unnaturally long life thanks to the genetic "gifts" of his former employer, Apocalypse. In his long life, Sinister has played puppetmaster in many of the X-Men's lives, especially that of Cyclops and Jean Grey. Sinister was a presence at the orphanage where Scott Summers was born and cloned Jean Grey, creating Madelyne Pryor, who would become Scott's wife and give birth to their child, who would grow up to become Cable, all in a convoluted attempt to stop the plans of Apocalypse.
It is also worth noting that the Essex name was shown on a suitcase collecting a genetic sample of Wolverine in the post-credits scene of X-Men: Apocalypse. In other words, the introduction of Mr. Sinister, especially with such a big star playing the character, could mean big things for the X-Men movies franchise going forward.