Once Upon a Time: Zelena Makes the "Ultimate Sacrifice" for the Heroes

The Black Fairy led Zelena down into the dwarf mines and antagonized her into a fight, which allowed the fairy to use the wicked witch's dark magic to transform the light magic fairy crystals into a weapon of dark magic destruction.

If Zelena hadn't learned anything from the encounter she might have taken Regina's harsh advice and returned back to Oz and left the heroes with absolutely no hope in countering the Black Fairy's power.

In an extremely rare instance of selflessness, Zelena destroyed her magic and effectively set the Black Fairy back at square one in her mission to destroy all light magic.

After seeing how the Black Fairy was able to whip Zelena and Regina -- two of the most powerful sorceresses in any of the realms -- around, Emma is going to need a weapon of epic proportions if she wants to destroy the source of all darkness.