Here's how Star Trek changed the villainous Khan from The Original Series for Star Trek Into Darkness. As Ambassador Spock (Leonard Nimoy) told the younger Spock (Zachary Quinto): "Khan Noonien Singh is the most dangerous adversary the Enterprise ever faced." Indeed, Khan ranks at the very top on the list of Star Trek villains and Admiral Kirk (William Shatner) bellowing the tyrant's name is arguably the most memorable and most imitated moment in all of Star Trek.
Portrayed by the legendary Ricardo Montalbán, Khan debuted in the Star Trek: TOS season 1 episode "Space Seed". He was the leader of a group of genetically augmented "supermen" who ruled Earth during the Eugenics Wars of the 1990s. When the world turned on them, Khan and his followers escaped the planet and placed themselves into cryosleep as their DY100 freighter traveled across the stars, only to encounter the Starship Enterprise in 2267. Once awakened, the charming but cunning Khan wasted little time in trying to take over the ship, only to be foiled by Kirk, whom he developed a grudging respect for. Kirk's solution was to deposit Khan and his people on the uncolonized planet Ceti Alpha V, which they could make into their new home. The Enterprise's historian, Lieutenant Marla McGivers (Madelyn Rue), fell in love with Khan and she joined him on his new conquest.
When producer Harve Bennett took over the Star Trek movie franchise, he watched TOS to familiarize himself with the series and he was struck by Khan, who stood out as a larger-than-life villain with a story cliffhanger worth returning to. Bennett chose Khan as the titular antagonist of Star Trek II: The Wrath of Khan, which revealed that the planet Kirk left him on became an uninhabitable wasteland and Khan blamed Kirk for his fate and for the death of his wife Marla. Khan stole the U.S.S. Reliant and the Genesis Device, which could terraform dead planets into an Earth-like paradise. Kirk and Khan engaged in a battle of wits that had tragic consequences; Spock sacrificed his life to save the Enterprise as Khan detonated the Genesis Device, which led to his own doom. Star Trek II is widely regarded as the best Star Trek movie; Montalbán returned to play the older, more maniacal, bare-chested conqueror, who quoted Moby-Dick as he tried to prove his superiority over Kirk.
For their sequel to their hit 2009 Star Trek reboot, director J.J. Abrams and his writers, Roberto Orci, Alex Kurtzman, and Damon Lindelof, gravitated to Khan as the villain for Star Trek Into Darkness. While much of the movie is about Captain Kirk (Chris Pine) discovering Starfleet Admiral Marcus (Peter Weller) was secretly trying to militarize Starfleet, the filmmakers grafted Khan into their story as the secret villain and chose to repeat many of the key story beats of Star Trek II.
Benedict Cumberbatch, hot from playing the title role in Sherlock, was cast as Commander John Harrison, who was later revealed to be Khan incognito. To preserve the secrecy of their "Mystery Box", Abrams and his team repeatedly denied that Khan was in Star Trek Into Darkness in the build-up to the film, which backfired when many fans and critics rejected the swerve and decried the film as a blatant remake of Star Trek II.
Cumberbatch's Khan is meant to be the same character played by Montalbán, only awoken from cryo-sleep 9 years earlier in the Kelvin Timeline. Although his version is obviously Caucasian when the character is believed to be a Sikh (while Montalbán was Mexican), Cumberbatch properly conveyed Khan's calculating malevolence; his version hews closer to the Khan in "Space Seed" than the older, crazed Khan in Star Trek II. Also, it was Admiral Marcus, not Kirk, who revived Khan; Marcus meant to exploit the genetically augmented warlord's savagery to benefit his vision of a militarized Starfleet. Since Khan is a technical genius, he built weapons for Marcus before he realized the Admiral would betray him and Khan tried to save his people from Marcus by destroying Starfleet.
Both versions of Khan are superhumanly strong but the key switch Star Trek Into Darkness made was Kirk dying to save the Enterprise so that Spock could defeat their adversary. In The Original Series, Kirk needed weapons against Khan but Spock, being a Vulcan, was physically a match and bested him in a fight by unleashing his own suppressed anger. Luckily, Khan's genetically altered blood possessed healing properties (something also not in TOS) and Dr. McCoy (Karl Urban) was able to use it to resurrect Kirk. While Montalbán's Khan perished in a fiery explosion, Cumberbatch's Khan was placed back in cryo-sleep so that the villain could possibly return in Star Trek 4 or a future sequel.