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—
"I mean, even 'because I wanted to' would be more information than I currently have. I already suspect that'll be the answer, but, I don't know, I can't... understand... any of it."
"I'm pretty sure he was not under orders from yet another master up the chain, we would have noticed by now even if he was exceptionally discreet about it. So. Because he wanted to."
"But I mean, sadism isn't unheard of amongst mortals, but most still notice that it's... not a good thing to do it to people who don't want it done to them?"
"I'm not sure, I guess basic empathy and maybe asking? I suppose there are some selection effects that might cause certain types of sadists to go into careers where they can more-or-less express it like law enforcement, so maybe I shouldn't be so surprised that the lack of a government would allow this."
"It's still confusing and terrible and a part of me wants a better explanation than 'he wanted to and had not an ounce of empathy in his body.'"
"I have no idea. 'It never occurred to me that other people existed or had feelings.' 'I thought I was god.' 'I thought I was fictional and so were all other people.' 'My kind needs to torture people to survive in addition to cutting things a lot.'"
"I am reasonably confident none of those are the truth. Especially the one suggesting he needs anything to survive. He's a fairy."
"Yeah, I know, I'm not particularly proud of the reasonableness of the part of me that wants a satisfactory answer."
"He doesn't care about other people," Promise says, "and he gets off on controlling and hurting them. I think that's all that's there."
And does—wait, she doesn't have surveillance equipment with her, does she. Well, they'll ask later.
"My altruistic impulses and curiosity should not be focusing on her in particular out of all of Thorn's vassals, probably."
"Six not counting the secret sites. Do you want me to send him to get your mother after this one?"