Board Thread:Show-Related Questions and Answers/@comment-8345449-20151204052326/@comment-27213286-20151205120902

When Nimue broke excalibur she did so to prevent it from being able to kill an immortal, this prevented the dagger (which is what Zelena was stabbed with) from being able to kill immortals. The dagger can transfer the Dark One's powers from one person to the next, but I believe it is the dagger wound once the immortality is transferred that kills the previous Dark One. This killing of the previous Dark One is what helps each new Dark One to embrace the darkness as killing is the darkest of all sins