The Insider.com

Regina ended up being her own worst enemy by inadvertently poisoning Henry in the penultimate episode of Once Upon A Time -- and the creators have a message for fans who are assuming they would never kill a kid.

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"On a show like this, to have real stakes, you have to be open to killing everybody," co-creator Edward Kitsis told TheInsider.com. "And that unpredictability is important because it adds to the stakes of the show."

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That takes center stage in Sunday's finale, titled A Land Without Magic, as Emma and Regina are forced to work together in order to save Henry's life.

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But that won't be the only unexpected development in the finale. "We really feel like there are some unexpected elements in the episode," Edward says. "It's not about dropping something into the finale that's completely new," co-creator Adam Horowitz adds. "It's a very trick dance you do to keep the show true to itself while also evolving it into something new for fans. I'm excited to see how they react."

Once Upon A Time airs Sunday at 8 p.m. on ABC.