Board Thread:General Discussion/@comment-5571357-20151016110421/@comment-3274978-20151016111738

There are many threads about this. It's simple: Her life force freed from her body when Gold "killed" her and then she released the magic inside her pendant. Thus, gaining her magic again. She finished the spell and followed Emma and Killian to the past, where she recomposed her physical body and killed Marian, assuming her identity.

I think the reason why her life force could leave her body is beacause of the pendant, which,as Glinda said, it's now her life. As long as the pendant exists, Zelene lives.