Disclaimer: All characters, places, settings, and story objects that bear any resemblance to the works of JK Rowling remain hers. Original characters are mine unless otherwise stated.
"Make the oath, Potter."
No one had ever dared to call him that, not even his or his father's worst enemies. Nobility didn't matter to her, not to this Hermione-and neither did wealth nor status. She readily ignored her own, even when her legacy seemed far greater and more important to Magic than his.
But in this place, he could feel their magical legacies move between the two of them like great shadowed beasts swimming under the Black Lake. The Black and Potter magics loomed behind him, and he could feel the presence of her own first-generation powers settle like a behemoth in the clearing. They were alone, but had the vastness of magic stirring in the air, stretching across generations.
She stepped forward, and he couldn't look at her.
He could barely lift his wand, or say the words. But she was waiting, and she was clearly scared of the unfamiliar magics moving between them (but I would never hurt you), and he'd give anything to take away her fear and pain. "I...I can't, Hermione. Please don't ask me to do this."
"Swear it on your magic right this instant that when this is over, you will send me back, Harry James Potter, or else I'll...I don't know what I'll do, just do it!" Her voice had a slightly-hysterical quality to it. That she didn't get his name right hurt more than he could ever admit.
"Or else what, Hermione? What else can you do to me that you haven't done already? What makes you think I owe you anything?!"
Why does it have to be you, and why do you have to do this?
And suddenly she's in his face, she's the closest she's ever been to him and he can't help but file away in the deepest part of his mind the scent of her hair, the thrum of her magic, and the way her eyes flash with defiance. He knows she had her wand pointed at his chin, but he couldn't seem to care. The Potter and Black magics heave, as if to answer the threat, or swallow her whole.
He could throw her in Azkaban without a trial for threatening a peer at wandpoint, but that would seem a mercy compared to what she was asking of him.
"I don't care what you think, or what I owe you. You make the oath now, Harry, or I will swear on my magic that I will watch this world burn even if I have to light it on fire myself!"
His eyes widen as he sees the progenitor magic manifest on her skin, like a slow-bubbling potion tinged with oil-slick colored light. His own family magic crackles like lightning around him, and he realizes that it's not just Magic but also Fate that is at work here. He has no choice. His destiny was calling. He must help her, even if it broke him in two.
He closes his eyes in defeat, and feels the bonds of honor, sacrifice, and love, so much love and longing wrap around him even as he agrees and the vow settles wordlessly in his core.
"I, Hadrien James Potter, son of Sirius Orion Black, Earl of Gryffindor, Viscount of Grimmauld, swear on my Magic to help Hermione Jeanne Dagworth-Granger, daughter of Agnieshka Dagworth, Duchess of Graystone, complete her mission to me, to herself, and to the ones she loves with all my magic, resources, and life. So mote it be."
He heard her sharp intake of breath at the wording, and felt a deep satisfaction that he had finally caught her off-guard, and that he didn't do exactly as she wanted him to. Her wand left his skin, and she pointed it at her sternum.
"I, Hermione Jane Granger, daughter of Monica Wilkins, swear on my Magic to help Hadrien James Potter, son of Sirius Orion Black, Earl of Gryffindor, Viscount of Grimmauld, defeat Voldemort and his Death Eaters in all their forms and schemes to subjugate Magic and her gifts until the prophecy is fulfilled. So mote it be."
Was that her real name? Her mother's real name? From where she came from?
Harry looked down to see a glowing band of light connect the center of his chest to hers, magic flowing freely between them.
The Gryffindor and Black family eidolons manifested beside him, the griffin and the basilisk bowing to the Duchess of Graystone in acknowledgment of the vow. Behind Hermione, he got his first look at the Graystone eidolon, the manifestation of her family line of magic. At first he thought it was a lion-then he saw the terrible beauty of its face, and the enormous wings. The creature bowed, and magic rushed around them and up into the sky.
He finally had her, irrevocably bound her-but when all is said and done, he was still going to lose her.