I think the “sorceress from another land” is Emma rather than Zelena. Emma will eventually travel to Oz via a tornado and unseat “the greatest evil” who is Zelena. Wait a minute. Isn’t Zelena already dead? Or is she?
We saw Zelena’s power is contained in her pendant and it opened up a time portal. I think Emma and Hook will first travel back in time to the Enchanted Forest. From there Emma will travel to Oz via a tornado and kill Zelena in the past, thereby defeating her once and for all.
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"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy