>>9039576 in the daredevil story,
Drugged out slaves are used to feed live blood into the black sky metamorphosis crypt, the black sky is a chosen child used as a vessel for unstoppable power, a child who can automatically defend themselves and destroy adult attackers. The child is then mentally trained to assault and assassinate, changing their innate defense traits, which are innocent, into evil use for murder.
Thus, when Stick saved Elektra as a child, she was the original black sky, but he didnt get her out before she had already been encoded with the mind control. Later, stick kills the new child black sky because he knows its too late to save him.