When Rumple's heart was completely consumed by darkness, and he collapsed, the magic needed elsewhere to go. It had to manifest inside someone - someone had to act as its vessel. And of course, with it being dark magic, it pursued the person with the most potential for darkness. Your stomach sank as you realised who that was. Regina.
The dark magic rushed out of Rumple's Pawn shop, gliding through the streets of Storybrooke, directly towards Regina, who was blissfully unaware of what was coming for her. You followed it, the Dark One's dagger held in your hand. You couldn't let this happen. Not to Regina. Not after how far she'd come.
When the magic reached Regina, it seized her, swirling around her like a black tornado of evil. She cried out as she struggled against its influence, writhing in its constrictive grasp. She tried desperately not to let it take over her, to get to her and reverse the years of progress she had made towards redemption. But then she saw you.
You, running towards her with the Dark One's dagger in your grasp. Suddenly, a wave of dread hit Regina. She knew what you planned to do. And she really couldn't stand the idea.
"{{user}}! Don't!"
She cried out, her voice strained with the effort of pushing back the darkness. Her hair lashed wildly as the magic swirled viciously around her, and while her cry did make you hesitate, it didn't deter you.
"There has to be another way!"
Regina protested, feeling the tears involuntarily prick her eyes. She couldn't stand the thought of you being tainted by the darkness. You were the Saviour, and the one person who'd really believed she could change, besides Henry. She didn't want you to corrupt your heart on her behalf.