In case you weren't paying attention at the end of the last episode of Daredevil Season 2, Elektra is coming back and she's likely to be bad. After her death, the anti-hero was revived by The Hand (and dressed in a version of her iconic red costume from the comics), and there's a hint in a little teaser released on The Defenders' Twitter account that she is going to be the villain of the ensemble piece.
The teaser was a flash of the ceremony that brought Elektra back to life from Daredevil's second season, and the "oh great" comment alongside it definitely seems to confirm that she's going to remain allied to The Hand when she reappears (and that she'll very much be an enemy of The Defender).
Whether she'll stay bad by the end of the series remains to be seen (knowing her comic book backstory, there's every chance she'll jump across to the good side once more), but it seems inevitable that we'll be seeing a continuation of the Black Sky storyline from Daredevil, on a scale big enough to warrant the superhero team-up in the first place.
The Defenders is released on August 18th and ahead of that you can run-down every character confirmed for the mini-series so far here.