Hermione waited until Harry left to take his first watch at the castle gates to corner Grindelwald about the cloak. He was in one of the armchairs across from the window, a book lying open on his lap but unread as he stared out over the valley.
'I need to set down some ground rules.' She said firmly, taking a seat in the chair opposite him. The Dark Wizard's attention snapped to her and she took a deep breath.
'Rules?' He queried, seemingly confused.
'Yes, rules.' Hermione confirmed. Grindelwald sneered.
'You can't try to steal Harry's cloak.' She decided to just be blunt, Grindelwald clearly already knew it was special. If he didn't, he'd figure out as soon as he saw it.
'It's one of the hallows, isn't it?' He confirmed and Hermione narrowed her eyes.
'That's none of your business.'
'Of all the hallows, I was least interested in the cloak.' He seemed unconcerned by how Hermione had just snapped at him. 'Albus always wanted the stone; he believed it would bring back his dead mother. I believed it would make building an army of inferi easier. I wanted the wand, which was everything I always thought it would be. I didn't see any point to the cloak then; why hide when you have an unbeatable wand? I only wanted it to complete the set and become immortal.'
'Of course, I found another way. I found some research by Morgana herself into a ritual to prevent aging and managed to complete the spell, then in the late 1920's I met Nicholas Flamel face to face. He may have been able to live forever but he was still aging, and I bragged about how I would be young forever. Of course, he soon woke me up to the reality of immortality; watching your allies age and die around you.' He was looking out of the window and she could almost see the losses playing out across his features.
'I believe this ritual will have a time frame before it runs out. It draws off a finite source, but I have no idea how long it will last. When it does end... I may age rapidly and die or just resume aging where I had left off. Either way, I have no reason to want immortality any longer.' He finally looked back to her.
'Well, it's still a rule. You're not allowed to steal, or even borrow Harry's cloak without his express permission.' She said decisively. He wouldn't be able to trick her into allowing him to get it. 'You're also not allowed to force Harry into giving it to you or letting you use it.'
'Anything else?' The dark wizard asked as if he was suffering through a long, patronizing experience. Hermione quickly wracked her brain to come up with anything else whilst she had his attention.
'No torture.' She offered, wincing as her voice went up at the end as if she was asking a question.
'Let's leave that one open to debate.' Grindelwald suggested. Then stood up and left before Hermione could stop her outraged spluttering.