If you have been there for Once Upon a Time since their first airing, October 23, 2011, you know Emma Swan went through a tragic turn of an events by entering the quite town of Storybrooke, she set a motion of events that will make her life change forever. Since she found out the stories of her childhood were true, then she found out something more dramatic, she was apart of the story. Since the first season ended they have gained mass views and ratings. Now at the ending of season 4 Emma does something that will turn the events of time. She becomes the Dark One, now how does this happen? You may ask, well this happens only after Henry restores the universe and breaks the spell that pulled and switched roles on Hero's and Villains. Rumpelstiltskin, the previous Dark One was running out of time, his last little bit of good was dimming and going out, he was going to lose his humanity. So they tried to take the darkness out of him and place it into Merlin's hat, but it was too strong, it tried to take over the apprentice, but Emma spelled it out of him.
When the Darkness tried to take over Regina, Emma knew what she had to do in order to save Regina and to help her keep her love and humanity, Emma knew she had to sacrifice her humanity and become the Dark One. So she plunged the Dark One's Dagger into the darkness that was surrounding Regina and absorbed the darkness inside herself, now the last thing that you see is Emma being lifted off and the Dagger falling and saying her name.