X-Men: Mister Sinister
This inclusion marks the second big villain of the X-Men universe, Mister Sinister. Sinister possesses enhanced durability and an extended lifespan thanks to Apocalypse. His powers grew as he used the genetic material of other mutants and applied it to himself given him telepathy, telekinesis, super human strength, and the ability to project concussive energy from his hands. Sinister also had control of the mutant group called the Marauders and frequently the devious acts he carried out were ordered by Apocalypse. His human name was Nathaniel Essex, and he was a gifted doctor albeit slightly screwed in the head. When he was changed by Apocalypse, he adopted the name "Sinister" due to his wife, Rebecca's dying words.
Doctor Nathaniel Essex was an incredibly smart individual but one day he came across the mutant group called the Marauders underground. The Marauders had collected freaks, fools, and all types of people from the streets of London. Essex hired these thugs to continue their devious acts so that he could perform experiments on the victims in the name of science. Eventually, Essex had awakened the immortal mutant, En Sabah Nur (Apocalypse), Nur was impressed by Essex work. Essex was so far gone to the point that he even began to experiment on his dead son, who was also a mutant. Essex's wife walked into his lab one day to see the sinister acts he was participating in, and just so happened to see their son in a test tube. Horrified by the image, she stormed out of her husband's lab. Essex quickly allied himself with Nur and the two saw eye to eye on many things. Essex took him to the Hellfire Club where Nur demonstrated his amazing power. Essex also had Cyclops and Jean Grey kidnapped and two pleaded with him to let them go. They continued to explain to him that if he keeps siding with Nur, the world will be destroyed. Essex returned to his lab to find that his wife had released all of his captives and that she had prematurely had their baby and she was now dying of stress. Her final words to her sick husband were " To me, you are...utterly...and contemptibly...sinister!" Following this, Essex took Nur's offer and was ordered to create a plague to destroy the weak of the world as Nur's first disciple, Pestilence. Essex was painfully transformed into an ageless being and began to call himself "Sinister".
As he emerged from Nur's machines, he explained that he willingly allowed his humanity to be stripped from his being all in the name of science. Cyclops escaped from his captivity to endlessly fire his optic beams at Sinister. Sinister wondered why all the sudden Cyclops had so much anger towards him all of the sudden, but then quickly he realized the hidden knowledge he had tucked away in the young man's mind long ago. When he was Essex, he came across Cyclops when he was a young boy named Scott Summers and he saw the hidden power buried deep within his being. Essex saw the same power in his brother Alex Summers (Havok) and kept a watchful eye on the two of them as they grew up and viewed them both as very powerful mutants, if not the most powerful of mutants. He figured their genetic coding held something special that he could use. Upon his creating of the plague that Nur ordered him to make, he manipulated it to only affect Nur, but when the plague was unleashed, Nur's immunity system quickly adapted to it. Nur asked why Sinister would defy him and he simply responded that it was ignorance and disrespectful to science. Nur then would return to hibernation but before so he stated to Sinister that he admired his strength but he would not put up with it again.
With Nur gone, Sinister turned his attention to finding the perfect mutant. In his attempts to find this perfect mutant he followed the Summers' bloodline but closely followed Scott Summers. Some time later, Sinister recruited Gambit to gather the Marauders and ordered the massacre of of the Morlocks. Many casualties were had on both sides but it eventually led to Gambit breaking his bond from Sinister and joining the X-Men. Sinister then took the dead body of Jean Grey and cloned it to make Madelyne Pryor so that Scott Summers would have a child with her which Sinister would view as the perfect mutant. The two got married, but Scott's heart didn't fully belong to Pryor and once as Jean returned from her stasis form, he then fell back in love with her immediately. The two did have a child however, and the child would be sent to the future. Eventually that child did return to the present timeline as Cable. Later on, Sinister would then be involved in Stryfe's attempt to assassinate Charles Xavier. Sinister traded Jean Grey and Cyclops to Stryfe in exchange for their DNA. However, Stryfe did not give Sinister their DNA, instead he gave Sinister the Legacy Virus which led to the deaths of many mutants.