After she was buried by Matt and Stick, The Hand dug her body out of the grave and placed it in a stone sarcophagus.
In the Daredevil Season 2 finale, Elektra gave her life to save Matt Murdock from The Hand during that climactic rooftop showdown.
What makes Elektra’s involvement in The Defenders more notable than the other supporting players announced so far is that when we last saw her, she was dead.
It was revealed earlier in the season that The Hand believed Elektra to be a Black Sky, an individual who was destined to become the organization’s leader.