Warning: this article contains major spoilers for The Defenders!
Marvel's The Defenders may have built up the hype to see the four superheroes together onscreen for the first time, but there's also been a lot of anticipation for the return of Elektra - and she's every bit as evil as you'd expect. Elodie Yung's legendary warrior is brought back from the dead thanks to Sigourney Weaver's Alexandra, and the show informs us exactly how she did it.
We've seen in both Daredevil season 2 and Iron Fist season 1 how the Hand brings dead people back to life, using that ancient stone bath and what looks like blood inside. In episode 3, 'Worst Behaviour,' we witness Elektra's resurrection, organized by Alexandra (two months before the events in the new series), and how they make "the substance" that brings them all back to life. That substance, it turns out, is made from dragon bone - a.k.a. "The Beast" - which the five leaders of the Hand stole from K'un-Lun after they defected and started their own criminal organization to fund their immortality.
Yep, Game of Thrones isn't the only fantasy series with dragons and for centuries the "heretics" have been using "the substance" to resurrect themselves and their followers. However, to bring back Elektra, The Hand's founders - Alexandra, Madame Gao, Murakami, Bakuto, and Sowande - had to use "the last of their resources" for the ceremony to work. As the leader of the Hand, Alexandra persuaded them that bringing back Elektra, or as she calls her "the Black Sky," would give them the weapon they needed to get more of "the substance" so they could continue to extend their lives as well as return to K'un-Lun. As Alexandra is dying, she put everything on the line to bring the assassin back.
Once Elektra emerges, reborn as it were, she is not the same person. She is a feral shadow of her former self, traumatized by her death and the emptiness of it. As Alexandra explains over two scenes:
"I know you're afraid but that will pass and so will the pain. This is your home now, we are your family. I have waited so long to meet you. Language will come back to you. So will your instincts but everything else I'm afraid it wasn't worth keeping anyway You were born, you lived and died, and what you saw on the other side, the darkness, the absence of everything, it's horrifying, isn't it? I've seen it too, more than once, and all I want in this life is to never see it again."
The "everything else" Alexandra is talking about is the memories of her former life. It's not explained exactly how Alexandra was able to erase her memories, as the Hand leaders and Harold Meachum remember their lives before their death, but it seems that a lot of manipulation is used in order to get their weapon in check.
Alexandra takes responsibility for this by refusing to give reborn Elektra any information about her former life with Matt Murdock or Stick, telling her stories of her power and purpose, and putting her through rigorous training in order to bring out the "Black Sky" in her. The ninja's inmate ability for violence and death is what the Hand are relying on to win their fight against the Defenders and with Alexandra's guidance, Evil Elektra lives up to their every expectation.
Under their orders, the warrior kills everyone in K'un-Lun while Iron Fist is finding himself in New York (in the first season of his solo show). She reveals this to Danny Rand at the end of the series,(while trying to force him to open a gateway to more dragon bones locked by a previous Iron Fist), telling him "you should have heard the way they screamed when I killed them." However, once Evil Elektra is confronted by Matt again, she starts becoming a bit of a liability as it becomes clear that her memories of love for him were not erased but merely suppressed. The more she interacts with Daredevil, the faster her memories come back but that doesn't mean she becomes good again.
While the assassin doesn't kill Matt when she has the chance, she does learn more from Alexandra - about "the substance" and why they need Iron Fist - and ultimately decides to kill her mentor, take over her position as leader of The Hand and stop being a slave to their needs. "I also had masters but I'm free now," she tells Danny, speaking not just about the criminal organization but also Stick and the Chaste who have also used her as a tool over the years. Evil Elektra reconnects with the person she was once before but gives herself over to the dark side of her personality. The only light inside her is brought out by Matt, but even though she knows who he is now she doesn't "care about good or bad" because she has the key to immortality and she wants to share that with him. That's why she never killed him, because she wants him to be by her side forever.
At the end of The Defenders, it's hinted that she died after the Midlands Circle Financial building gets detonated but, of course, it doesn't mean that she's dead. If Daredevil can make it out battered but alive it seems pretty obvious that she survived too - but what kind of Elektra will he be dealing with?
If the comic books are anything to go by, Elektra will continue to cross the line when it comes to good and bad. In Enemy of the State: Wolverine, she allows herself to be killed and resurrected by The Hand, making them think she has been brainwashed to do their bidding, but really she's in charge of all her faculties and used it as an opportunity to infiltrate their ranks and takes down the leadership. It's during this comic book run - written by Mark Millar - and while pretending to be controlled by The Hand that she leads an assault on S.H.I.E.L.D., killing multiple agents. Clearly, she has no qualms with doing whatever's necessary to win, even if it means killing the "good guys," and if she returns in Daredevil season 3 it's safe to say she's going to be causing even more trouble for both Matt Murdock and Daredevil.