The plan is refined and strengthened, contingencies are set to deal with various ways Thorn could've strengthened his defences, Mortal receives a very interesting email concerning one of their mother's contingency plans—namely that her assets have all been either frozen or transferred to Mortal themself, so Thorn doesn't have access to them -, and when Mortal and Promise judge there's nothing more to be gained from further planning they (eat dinner, sleep again, have breakfast, go over the plan once more when it's not completely fresh in their minds so they see if they come up with anything new, purchase Promise a mobile phone and a power generator to charge its battery in fairyland, eat lunch while Mortal teaches Promise how to use that, and) part ways.
On this side of the veil, Mortal gets to work. The first step: getting at least two safe houses, one for their HQ and the other for contact with the place near Thorn's court. They end up getting five, for redundancy's sake. The main HQ is near Seattle, the other four safe houses are in Greece, Russia, Japan, and Argentina. That, and getting the necessary existing equipment, is the easy part—you end up with contacts of the relevant sort when the bulk of your wealth comes from the kind of thing you can get with sorcery. The hard part is getting the various specific bits of technology that don't exist yet, including the the many types of trap and ammunition they'll need. Processing fairy voices with software turns out to be a dead end—apparently they're weird magical superpositions of sounds that make software go ?!?!?!?!?—but everything else, as agreed upon, can be made to spec nicely enough.
It'll take a couple of weeks beyond the one month for everything to be ready. Mortal hires someone who talks very fast.
And after the ball's going, there's not actually much for them to do with their time. They fret about details of the plan, order more redundant pieces of stuff (especially the to-spec stuff, not being mass-produced means they'd better have a lot of it to start with in case anything goes wrong), and have various antsy and anxious and calm and relaxed and terrified and panicked moods. A month is a long time...
At one point it occurs to Mortal that Promise might've decided to run away and not help, and then they'd never see her again and that would be terrible, and why would it be terrible anyway? It's just some fairy, fairies are evil, one must remember that. Even though she wasn't, of course, she was smart and resourceful and moral and ridiculously hot, and if they never see her again she'll never order them again and the tingly feelings won't ever happen again. Except what the heck, what are they even thinking? The answer, of course, is that they want to see Promise again. Why? To save their mother, of course. The only reason being ordered like that felt good was because Science. Of course. Of course.
The month passes—
"Might give fairies who are sad about the abolition of slavery some new way to entertain themselves."
"Prrrrrobably not, but impulses that require nonconsensual suffering aren't ones I'm very inclined to allow people to indulge in, I think."
"Alright, so, do I go with you to your tree or should I wait for you to gate to the mortal world?"
The gate is a ways away from the tree; the tree isn't visible as one emerges. It's walkable, though. Promise leads Mortal to it.
It is huge and shady and beautiful and the trunk opens when Promise approaches.
It is not very lived-in yet, but the wood itself is shaped furnitureishly. Yellow's in it, perched in the bottom of three bed-nooks around the wall up high on the trunk. There is fairy food around.
"This is a really nice place. What kind of control do you have over the shape and growth? Is the food yours, or Yellow's, or should I worry about claims?"
"I can make it be shaped however I want. It grows at normal tree speed, unless I do sorcery to it, but I can move what wood is there as I like. The food's foraged, except for the haws, which are fully mine. If you want any non-haw fairy food one of us should feed you."
She nods. "Can you make new rooms? It might be useful to have an extension of HQ here."
"Yeah. It's just not very big in here yet so I didn't subdivide the space. I can grow it more."
"It's a very nice tree. Is your original tree still standing where you left it? I don't know if it can be used as any sort of leverage against you, I'm not very clear on your relationship with it..."
"It's still there. I don't need that individual tree, but I was worried about it when it was the only one."
"I can only do my-tree stuff with descendants of the original tree, and I didn't think to plant any extras before Thorn got me." She strokes the wood wall.
"Right, I mean, what's the your-tree stuff you can do with it? Shape it however you want, I expect you have pretty strong claim over food generated by it..."