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—
"Mmmm enough time for a power nap, I was next up in the sleeping schedule and would prefer it didn't bother me while in the thick of it, any objections to my doing that now?"
Ahead he goes, and he only tosses and turns in anxiety for about half an hour before falling asleep.
He sets an alarm to wake him up thirty minutes before Thorn's estimated time of arrival.
"It'll be over soon. Although we'll have to find an outside sorcerer to turn him into a sparrow."
"It might not if he really wanted to be a sparrow for some reason but he won't."
"Edge cases. I see. That's... interesting." She shakes her head. "Anyway, is there a particular reason why we'd need to turn him into a sparrow? Not that I'm feeling particularly merciful, but we could just store him away somewhere."
"If he's a sparrow he can't speak or do sorcery and he's more storeable."
"Another planet, like Earth but larger and made of gas, we should probably not actually drop him in Jupiter itself but one of its moons—" She opens a browser on a side window to get relevant pictures of the Solar System and Jupiter and its moons. "...that might be a bit evil to him though, there's no breathable air there and it's below-freezing cold and while I'm not feeling merciful I don't particularly want to torture him unless we have no other options. We could just store him somewhere on Earth itself if we can't find a willing sorcerer..."
"If we store him on Earth he can't be a sparrow and he will be able to talk, but it's a good backup if we can't turn him into a sparrow."