"Wait," says Promise, apologetically. It's gentler than 'stop'; Elwing will still be able to breathe. "I'm very sorry about this but I need the Silmaril. Exclusively truthfully and exclusively to me, tell me what I need to know to take it away without any fuss."
She looks confused, then angry, then terrified. Then - "it's in a locked box in my room. I wear the key on my necklace. It glows even in the box, it doesn't glow too visibly if you wrap it with thick fabrics before you put it in the box and wrap the box. Everyone will try to stop you. I could tell them you're allowed but I don't think they'd believe I wasn't being somehow coerced, there'd certainly be a fuss. If you flew out over the water with it I'm not sure anyone'd realize what was going on, not if I told them I'd just given you some food for your people or something bulky like that."
"Will people notice right away if you're missing the key? How confident are you that you can sell the 'food' story until I'm out of bow range?"
"I'm really really sorry about this. How heavy is the box when wrapped up enough not to be identifiable?"
"Carrying that I can fly maybe forty-five miles an hour. Can you reliably keep anybody from shooting at me long enough for me to get away?"
"Is there anything else you keep there that you could invent an excuse to show me? Do you have a window, would anyone be watching the window from the outside?"
Damn the lack of sorcery. "Could you realistically go and get it so I wouldn't have to go to your room or would that be just as conspicuous?"
"That would be less so, but only a little less so, and someone might stop me and see that it's a Silmaril. If you went up with me to see it, and then out through the window? And the people who I told that you were going up to see it were in the palace, and I ordered them to a secret meeting before a cry went out that you were flying off?"
"To the extent this will be inconspicuous to people outside you may pace in this room."
"I am trying to get the job done as gently as possible. I don't want to have to do this to anyone besides you or micromanage you uncomofortably, but if I have to, I can. I recommend cooperating."
Sigh. "Which do you think has the best chance of success by my standards?"