I think Chernabog was the original dark one. Blue Fairy enchanted a dagger so that he could be killed. However, Chernabog was so evil that his evilness couldn’t be fully destroyed, it was contained in the dagger and transferred to the guy who stabbed him. The cycle continued up to dear Rumple. So long as the dagger remains, Chernabog is not fully destroyed. Think Sauron and the One ring.
In an Adam tweet, it’s implied that they won’t be splitting the characters up between two separate lands again.
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All magic comes with a price!
Keeper of Felix