Remember back in Season 3 when everyone in Storybrooke suffered a severe case of memory loss thanks to the Wicked Witch? Well, based on what we just saw in the Once Upon a Time Season 5 premiere, it's kinda sorta definitely happening again since no one can remember what happened in Camelot. Well, everyone besides Emma, it seems, who is now in full-on Dark Swan mode at this point in time. So whatever it was that ended up happening in Camelot, it's safe to say that it resulted in Emma giving into her Dark One nature. Which is too bad, considering that she seemed to be heading in the right direction before the one-year flash forward.
The gang had arrived just in time (courtesy of Granny's flying diner) to prevent Emma from killing Merida and they didn't even have to control her with the dagger to do it, which meant that there was still hope of saving her. Then King Arthur showed up and invited them to Camelot so they could help track down Merlin together. And then… somehow… it all goes very, very wrong. Obviously, I think we can assume that they didn't end up finding Merlin, or if they did, then Emma was already beyond the point of wanting his help. But, other than that, we're just as clueless as everyone else.
We know that Season 5 will provide us with some much needed background into the Dark Ones' history. And since we now know that the dagger is an extension of Excalibur, I think it's safe to assume that these two things hold the key to banishing the darkness and its power for good. But given that Emma was still in possession of the dagger, it's clear that these two pieces have yet to be reunited. Perhaps when they tried to do so, a dark self preservation in Emma kicked in and transformed her into Dark Swan. That could also explain how she came into possession of the dagger again.
If my theory is right, then in order to defeat the darkness, Emma must be willing to give up control of her powers and be the one to piece the dagger and sword back together. This would require her to hold the dagger once more. But what if, once Regina handed it over, Emma had a change of heart, decided the power was too important to her, and kept the dagger for herself, so that she could guarantee her powers wouldn't be taken away.
It's also worth noting that the seventh episode of the season is titled "Nimue," which translates to "Lady of the Lake." In history, this character is a known protector of Excalibur, so it's safe to say the Camelot gang will stumble upon her at some point during their journey. Perhaps she's even the one who informs them of what combining the sword and dagger would do. And considering what Emma ends up becoming, she may not have liked what she learned.
Images: ABC; stunningcaptainswan/Tumblr