Hermione always expected Ron to explode. It was just not in his person to even think about this idea before reacting. He was taught all his life that you do not take unbreakable vows, placing them in the same category of spells as the unforgivables. She hated when Ron acted without thinking, which is why she was really surprised by his response.
"I'll do it."
Hermione had to sit down. Harry and her had discussed the best way to break the news to Ron in the hallway and decided that it's best to just tell him as soon as they enter his room. She already had a list of reasons that she would use. Harry however realized that asking Ron to vow to follow Hermione was probably going to elate his friend. Ever since that trip to the Lovegood house, he knew that Ron was in love with Hermione, and not only that, he also knew that Ron knew that Ron was in love with Hermione. To give Hermione some time to catch up with this and to prevent the discussion from going in this direction, he decided to point out the obvious problem.
"Well, that settles that.", said Harry, "I have to wonder though, how exactly do they plan to do this? They have to give us our wands before we make the oath because we need to cast the vow. You have your wand Hermione, but since you are in both vows you cannot cast the spell. What's to stop the person casting the vow from using the wand to escape?"
"Harry, it's obvious." Surprisingly, the answer came from Ron. "They have the other two as hostages."
"He's right", thought Harry. "So, when do we do this? Do we need to go out and call the guards?"
"I think they already know Harry.", said Hermione. "They are keeping us under surveillance all the time.". As soon as she said that, the door opened and Paul, Piotr, Radebe and ten other guards walked in the room. Radebe was carrying the box Hermione recognized as the wand box.
"Good Miss Granger, good. You continue to perform at your level.", Paul gestured to Radebe who brought the box in front of Harry. "Mister Potter, if you would be so kind to point out your wand."
Harry didn't really have a wand in that box so he just pointed out the blackthorn wand Ron gave him. The wand was less powerful, but he wasn't really worried. Worst case, the vow he casts will not be as unbreakable as the name would suggest. Radebe then offered the box to Ron who pointed out his willow wand.
"Good. Gentleman, who would like to be the first to make the vow?"
"I'll do it.", said Harry. "I don't even know how to cast the spell. I'll let Ron cast first so I can pick it up."
"Wait!", Hermione was slowly recovering from her shock. "We need to sort out the details. We need to agree on the wording of the vow.". She looked at Paul, making a conscious effort not to look at Piotr at any point. "Would something like this be acceptable to you: Will you, Harry, obey my every order until Tom Marvolo Riddle, the current dark lord, is defeated?"
Paul immediately picked up on the reason behind this, but the Inquisition was here during the reign of the previous dark lord and will be here when the next one comes around. Having control of Potter would be a great asset. Especially if Potter becomes the next dark lord. On the other hand, his work on Miss Granger might be damaged if he pushes this.
"Yes, that is acceptable.", Paul held out his hand to silence Piotr's objections. "But you will say: fully and finally defeated and killed. We are aware of the several times Harry here has defeated him already. Somehow it doesn't seem to stick." He nodded to Radebe who took the two wands out from the box. "But first, let's start with you Miss Granger.". The tension in the room increased visibly. "Since these two gentlemen will be bound to you, I'm going to need you to promise to follow my orders. You must also promise to keep our secrets safe."
Once more in a very short time span Hermione was shocked. She had expected much more. He could have easily forced her to take an unbreakable vow as well. Even as a promise, it was surprisingly reasonable. He didn't even ask her to follow the Inquisition, but rather him personally. There were also no conditions placed on how she used her power over Harry and Ron. After her experience before the interrogation, it was obvious to Hermione now that Paul was manipulating her. Playing with her mind and trying to trick her into something. But try as she might, she couldn't figure out what that something was. If he was trying to win her over to his side, he could have just done that forcefully.
"I promise.", said Hermione.
"Excellent! Let's continue on then. Miss Granger, you are more qualified than us. If you would be so kind."
Hermione stood up and walked next to Harry. "Ron will need his wand.". The young deacon handed the wand over to the redhead. "Harry, kneel with me and take my hand. Ron, put your wand so that the tip is touching our hands and focus on the spell.". As Ron's wand approached their hands, a slight fiery glow emanated from it.
"Will you, Harry, obey my every order until Tom Marvolo Riddle, the current dark lord, is fully and finally defeated and killed?"
"Yes.". A thin stream of fire burst out from the top of Ron's wand weaving around their hands. Harry was now bound to Hermione. The stream of fire disappeared into their hands and slowly Harry let go of her. He didn't feel like anything was different. He stood up and Ron quickly replaced him next to Hermione. Very carefully, mindful of his injury, Hermione took Ron's hand. "There is no specific incantation Harry. The spell works by binding bits of our magic together. All you have to do is focus on being the bridge between Ron's and my magic.". Harry placed the tip of his wand next to their hands and focused. Quickly the top of his wand started to glow.
"Will you, Ron, obey my every order until Tom Marvolo Riddle, the current dark lord, is fully and finally defeated and killed?"
"Yes.". A thin stream of fire burst out from the top of Harry's wand weaving around their hands. As it did before, the stream disappeared into their hands. Unlike Harry, Ron wasn't going to let Hermione's hand unless he had to. Ron quickly stood up and used their joined hands to offer Hermione support while she got up.
"Now that this is done, what happens next?", asked Harry.
"Now, ", said Hermione, "we destroy Malfoy Manor."