The Birdcage has six hundred inmates. This won't be instant. But they're being let in to cell block W a few at a time, so at least everything on this side is orderly. A few people try lying about their names, but it quickly becomes common knowledge that this doesn't work. Very soon everyone ends up in Fairyland.
And then Promise's bodyguard goes invisibly through, Promise follows and collects the flattener she hid in the crystal, and she leads them all to Hawthorn.
Everyone else finds Hawthorn preferable to the Birdcage (mostly because it's not the Birdcage), but a large majority are curious about Promise's conditions for returning to Bet.
"I'm going to want to interview each of you," Promise says when she has them all assembled, "learn your cape names if you prefer them, learn your powers, learn what you want to do with yourselves - including if that's 'go home'. It's possible some of you cannot be made safe to my standards of safety to go back to Bet, but that probably won't apply to most people. You are also welcome to stay, and either lounge around the colony or volunteer to help me with constructive projects of one sort or another; I have a list of things that I'll post publicly after I've reshuffled the priorities in light of the interviews. If you are interested in helping me organize the other people here, raise your hands?"
Hands go up. All the former cell block leaders present, and a few per block who weren't. One block leader, a short man introducing himself as Teacher, volunteers his students to help with the construction.
The look Promise gives Teacher is eloquent. "Ah," she says. "There you are. You can be interviewed first."
She starts a list of interview time slots (in Hawthorn time), names some things that should increase priority, estimates a ballpark of ten minutes per person on average, and sends the volunteers except Teacher to prioritize everybody.
Teacher shows up first, looking as inoffensive as supervillainly possible.
"You're why I finally decided to empty the Birdcage," Promise remarks, writing Teacher and his real name.
She's not exactly hiding that this isn't because she thinks he was unjustly sent there.
"Summarize your power for me, please, I have it only secondhand."
Nobody comes to me unless desperate. There were a lot of desperates in the Birdcage."
Promise jots down notes. "Summarize the states of your current 'students'."
They are all still themselves, but many become more passive than before and some have lost noticeable amounts of intelligence."
"It's always a general power. Touch, willpower, a language, vision in another wavelength. But the end result is what I and the student expect it to be. I might be able to give someone the ability to know people's real names, if that's what you're asking."
"That is not what I am asking. I do not plan to use your power for anything except, possibly, incapacitating other fairies who are too immortal to be permanently dealt with in other ways, and for the foreseeable future I'm just avoiding other fairies entirely. I just want a sense of what I'd be working with in some kind of emergency. Use of your power is purely voluntary on your part?"
Whether you amend that or not, yes, I would."
"They're all capable of expressing that in the interviews themselves?" Promise asks. She adds Teacher to a separate list.
"They are. Almost all of them will be able to express opinions even without my power helping them. I can remove it early if you want to interview them in their natural state."
"They regain most of what they lost, but lose everything they gained. They'll end up with less independence or willpower or whatever the trade was than they started with. And they will go to some lengths to get the powers back, most of them. This may be an effect of my power or just of becoming accustomed to having it, but it's an effect one way or another."
"Noted. Please list everyone I have who has been one of your students by cape name."
He lists about two dozen. It's all those and only those who arrived from his cell block.