Marvel's The Defenders isn't just a crossover between Marvel's Netflix series heroes, it's a culmination of storylines that have been running through the separate solo series ever since Daredevil Season One.
It's revealed in early trailers for The Defenders that The Hand would be the threat that brings the four New York City superheroes together, but the exact nature of that threat has remained a mystery. There have been clues sprinkled throughout both Daredevil seasons and Iron Fist, but in The Defenders, the larger picture comes into focus with one big event: the resurrection of Elektra as The Black Sky.
Read on for an explanation of how Elektra returns, and The Hand's evil plan in Marvel's The Defenders. Be warned: MAJOR SPOILERS FOLLOW!
In Daredevil we saw The Hand at first working through Wilson Fisk and Nobu, and later through Madame Gao and a resurrected Nobu, to acquire a specific piece of property. In The Defenders, we learn that property is Midland Circle, the corporate building where the Defenders first meet up and fight as a team after investigating their respective connections to The Hand.
During the course of The Defenders, it's revealed that Midland Circle is actually built over the massive hole that Daredevil and Elektra discovered in Daredevil Season Two, and at the bottom of that hole is a gateway that can only be opened by Danny Rand's Iron Fist. Danny is tricked into breaking the portal open while battling The Black Sky, and is transported to a graveyard where the remains of a giant beast are being mined by The Hand -- supposedly a serpent like Shou-Lao The Undying, source of Danny's Iron Fist power.
The slivers of bone The Hand is mining is said to be the source of "The Substance," the oil-like material that the leaders of The Hand have used to cheat death for centuries. Since their supply was depleted, The Hand leaders were vulnerable, but having a significant source of The Substance would allow them to restock their greatest power source.
Needless to say, though... things don't go at all according to plan.
If you haven't guessed by now, Elektra comes back from the dead when The Hand uses The Substance to resurrect her after she was killed by Nobu in the climactic scene of Daredevil Season Two.
Specifically, Elektra is resurrected by Hand leader Alexandra (Sigourney Weaver) in order to capture the Iron Fist. Alexandra was motivated by the fact that, after centuries of living, she was finally succumbing to death. Since Alexandra was the one who used the last of The Substance to bring back Elektra, she found herself under pressure from the other four fingers of The Hand leadership to deliver Iron Fist.
After being resurrected, Elektra is mostly a primal empty vessel, with no memory of her former life. Over time, however, she begins to regain her memory and personality -- especially after she comes into contact with her former love, Matt Murdock/Daredevil. Eventually, Elektra regains enough of herself to kill Alexandra and declare herself the new leader of The Hand.
In the end, Elektra/Black Sky leads The Hand under New York City and through the portal to the caves under K'un-Lun to retrieve more of The Substance. However, the Defenders prevent them form escaping with it, and Elektra is last seen being buried alive with Daredevil as Midland Circle is brought down by explosives.
A final scene sees Matt Murdock having survived that ordeal with great injury, but there's no word yet on what Elektra's fate is.
Marvel's The Defenders is currently streaming on Netflix. The four solo hero series will each get new seasons in the coming year, with The Punisher expected this fall.