Just something cool and interesting taken from the comics. It doesn't have to be Mephisto necessarily.
Black Sky was a McGuffin really.
Elektra was hit-or-miss. Like, back in
Daredevil, she had some pretty funny moments when she wasn't being bloodthirsty. But there were also a
number of moments where there's too much "Uhh...what?" going on with her and the Hand for me to care. If there had been more foresight during the writing stage of
Daredevil season 2 for what they were going to do with
The Defenders, this could've been pulled off better, but I really wasn't into the Hand as depicted there at all, if only because they were the villains during the part of the season where Matt was being unnecessarily self-destructive (which is the reason I like to pretend
Daredevil season 2 ended at episode 6).
(On a related note, I also don't think anything with present day Matt has ever reflected the kind of almost last-minute "Turns out I actually love you, Elektra" decisions that the writers make to show him get really emotional over her in the climaxes of both
Daredevil season 2 and
The Defenders; a lot about Elektra and her relationship with Matt along with
the entirety of the Hand is really messy.)
I would say that the writers did not have any real idea when they started, as to how the
Defenders story would play out. The
Daredevil showrunners seem to have had one plan for The Hand and Elektra, and then it got really messy trying to also add in all the stuff established of the Hand over in
Iron Fist.