We all know the story: an ordinary orphan (the hero) is told by a stranger that they have an extraordinary destiny in another land, a land of magic and wonder, a land that needs saving from the forces of evil. Not only does this magical realm need saving from darkness, but the fight against that darkness is the reason why the hero became an orphan in the first place. As with every hero's journey, the orphan refuses to believe or accept their destiny, but eventually they embrace magic and defeat the darkness by having Voldemort shoot himself in the face with his own spell.
Wait, are we talking about Harry Potter or Once Upon A Time here? Well, in the case of Merlin and the Dark Swan, it's both.
As the Dark Swan storyline has progressed, her behavior has been more and more confusing, and we've found ourselves asking, "Why did Emma hurt Henry? Why did she erase everyone's memories? And whose heart did she use to cast the curse?" That last question is the one which will explain everything. After all, you have to crush the heart of the thing you love most to cast such a curse, and show creators Eddy Kitsis and Adam Horowitz have confirmed that Emma killed someone in order to do it. And yet Henry and Hook, the two most likely candidates, are still alive. (And would she really crush Henry's heart anyway? Hell no!) Her parents are still alive; Emma herself is still alive. So what's going on here? Who died?
At this point, all we have to go off of is that it's someone who was alive in Camelot but has been missing from Storybrooke. So far, the only candidate is Merlin. And not only is Merlin dead, but he asked Emma to kill him and go all Dark Swan on everyone in order to keep their secret plan in motion. (This is where the Harry Potter thing comes into play and I explain what I'm talking about.)
Image courtesy ABC Studios
We know from the press release and promo for "Nimue" that Emma and Merlin are going on a quest and that Emma comes face to face with the same Dark One who imprisoned Merlin as a tree. (Some people believe this person is named Vortigan based off the episode credits and because the actor who played Vortigan apparently said he was the Dark One, though I admit that I cannot find proof of this and I'm going only off of what I've heard through the grapevine. If he did say that, I think this is just a red herring to throw us off the trail.) The first Dark One was Nimue, Emma is going to summon her, and whatever happens in that meeting is what sets Merlin and Emma's plans into motion. Merlin will realize that Emma is going to have to play evil in order to reunite Excalibur and defeat the darkness. (Said another way, Dumbledore will realize that Snape is going to have to play evil in order to defeat Voldemort.) And so Emma will kill Merlin as a way of proving herself to the darkness, pretending she wants to reunite Excalibur to destroy light magic, but in reality she's doing all of this to destroy the darkness.
Eventually Emma's true intentions will come to light, Nimue/the darkness will be very angry, and in its attempt to kill Emma it will misunderstand the nature of her magic because true evil cannot understand true love... just like Voldemort. And, just like Voldemort, the darkness will end up shooting itself in the face with its own spell.
Image courtesy ABC Studios
But how could Emma use Merlin's heart to cast the curse if you have to use the heart of the thing you love most in order to do it? Well, thanks to Pan's use of Felix in Season 3, we know that the thing you love most doesn't have to be romantic love. It can also mean friendship and loyalty. Merlin will be Emma's strongest compatriot, supporting her because of his unwavering belief that she is the one who will defeat the darkness.
If you have doubts about this, well, that's okay. I don't expect everyone to agree with my theory. But let's not forget that young Emma made several Harry Potter references during the Frozen storyline. Consider also that Emma and Harry Potter share striking similarities, as summed up in my intro paragraph. And another thing: when Dopey went beyond the town line, he turned into a tree, just like Merlin. This isn't a coincidence. It happened because Merlin's heart was used to cast the curse.
How this will lead to the journey to the Underworld, I don't know, and I'm not even sure that it will play out the same way. But what I am sure of is that Emma will be the Woman Who Lived, freed from the darkness without having to die to be rid of it.