From strands of your parents' hair, I made the most powerful potion in all the realm. So powerful, that, when I created the Dark Curse, I placed a single drop on the parchment. Just a little safety valve.
Oddly enough, Mr. Gold and Emma are in the pawnshop both when he first claims to have created the curse and when he claims the Black Fairy has. The misinformation, however, was not addressed by either of them.
Mulan: How did you end up here in your cursed state? Aurora: You're not the only one who knows about sacrifice.
It is revealed on many occasions that Regina is not among the two most powerful. Even her own sister, Zelena, is later revealed to be more powerful than her and their mother.
From Rich to Poor
Robin Hood: I ever tell you about how I met Marian? I stole her father's horse. [...] Will Scarlet: And that night, you woke to an arrow pointed at your head. Robin Hood: Her family, she said, poor farmers. [...] I didn't say a word as she rode off that night. And the next day, I returned to her cottage-- Will Scarlet: With two of your own horses as a gift to her family. From that day forward you never stole for yourself again. From that day forward, you only stole from the rich and gave to the poor.
It is revealed seven episodes later, in the seventh episode of the fifth season, that Merlin tethered the Darkness to the dagger, rather than to the human soul, Nimue, who had welcomed the Darkness into her soul. The aforementioned battle was a mere verbal confrontation with Nimue to try and prevent her fall into Darkness which had nothing to do with it consuming all the realms, but rather her individual soul.
My family may be troubled dearie, but we all know that a heart is a precious thing, and I cast a protection spell on mine eons ago.
It was revealed one-hundred and two episodes earlier, in the second episode of the first season, that Regina did nothing of the sort. What did happen is that Henry tore out Emma's story out of the book so Regina wouldn't know she was the Savior.
Well that's just the thing, Miss Swan. The curse. That Dark Curse. It, and all of the darkness you've ever faced, was born out of one twisted soul. The Black Fairy. And unless we try and stop her, she is going to bring far worse.
It is revealed three episodes later, in the nineteenth episode of the sixth season, that the Black Fairy didn't turn dark until centuries after Darkness was born. Many of the dark things Emma had faced were therefore not born from Fiona, including but not limited to: