Elektra Natchios is a major character in the Marvel Cinematic Universe, serving as one of the tritagonists of Daredevil's second season, and the secondary antagonist of The Defenders. She is a skilled assassin who, while initially a morally gray anti-hero who helps Murdock against the Hand, is later resurrected by them as "The Black Sky" and becomes one of the principle foes of the Defenders.
Elektra is an assassin who was trained under Stick, (the same member from the Chaste who later trained Matt Murdock). She was tasked with bringing Murdock into the Chaste, but fell in love with him while doing so, and then abandoned him when he refused to kill Rocco Sweeney (the man who had killed Matt's father), on her behalf. She returned to his life after years of absence, Murdock reluctantly accepting her assistance against the Hand in New York City under the condition she kills no one, a promise she broke when she killed a young Hand initiate to save Matt.
Later, she was revealed to potentially be the Black Sky, the one who is supposed to lead The Hand. However, Elektra chose to help Murdock defeat The Hand, who vowed to kill Daredevil for meddling and taking away their destined leader. Although she and Daredevil had promised to start a new life together, this was denied them when Elektra was killed in the final confrontation with Nobu Yoshioka. Shortly afterward, The Hand retrieved Elektra's corpse from the grave and placed it in a stone chamber to be exposed to the Resurrection Elixir.
After her resurrection, Hand leader Alexandra Reid manipulated Elektra and took her under her wing, re-training her to truly become the Black Sky. After becoming the Hand's personal weapon, Elektra fought Murdock and his new allies the Defenders, which reawakened her past self through her interactions with Matt/Daredevil. Striking out on her own, she spent some time wandering New York City trying to remember who she was in full, culminating in her going to Matt Murdock's apartment and sleeping in his bed, trying to remember the intimate relationship they'd once shared.
After breaking free and taking over the Hand by killing Reid, Elektra descended into nihilistic madness, helping the remaining Hand leaders return to K'un-Lun no matter who had to die along the way. In the battle under the Midland Circle building that followed, Daredevil fought Elektra and tried desperately to get her to renounce the Hand, but Elektra rebuffed his every effort, consumed in nihilism and hatred. But at the last minute though, he seemed to finally get through to her, only for both of them to then be buried under the Midland Circle building when it was destroyed. Though Matt was later revealed to have survived, Elektra's fate is presently unknown.
Elektra helping Aleaxndra Reid interrogate Stick.
Elektra being threatened by Alexandra.
Elektra betraying Alexandra.
Elektra stabbing Alexandra to death before taking over the Hand.