If you watched the second season of Daredevil, then that imagery will look very familiar to you. It is obvious that this is teasing the return of Elektra. The assassin was killed in the final episode of the second season but her body was snatched away and locked in a coffin for purposes that are yet unkown. Elodie Yung, who plays Elektra, basically confirmed her return. It was known that the character would be appearing in The Defenders but to what extent and purposes is still unknown.