"Arguable." She's holding a little black leather-bound book, which is glowing and vibrating violently. "I have higher priorities. Give me my locket and you can have Dumbledore's stupid firebird back. Teach me the Fidelius and you can have your idiot godson too. You have forty-five seconds." 

What. What. You WHAT--

"First of all, die in several fires. Second of all, forty-five seconds to decide or to teach you a proprietary spell most people need six months of practice for. Third of all, fuck you and the walking corpse you rode in on. Fourth of all--"  

"First thing, Sirius please, didn't Harry tell you I'm still alive in here?" 

Wow, what the fuck, Tom hasn't let her actually talk for real since she was seven and a half. She's been trying every ten to fifteen seconds the entire time anyway, of course, because of who she fundamentally is as a person, and wasn't planning to stop even if it never worked for the next fifty years, but she did not actually expect it to. 

This fact is... not helping her blind terror even a little, actually. She does not want to find out what happens to you in the afterlife that Harry seems so sure exists if you die with your soul still stapled to the greatest evil of the generation. 

Sirius now has several additional questions they don't have time for. 

Alright, snap judgment: would Voldemort try this strategy if it was a lie? He certainly could, he is a famously good liar, but the question is whether he would. Pretending to be a terrified thirteen-year-old girl and saying please to someone who knows exactly who he is seems rather like it might strain his pride beyond reasonable limits, given, you know, his entire everything. 

"Fawkes first, locket second, then you oathswear--" this word is not typically used in wizarding English to mean anything less than a real magical Unbreakable Vow-- "that you haven't lied to me and won't renege and then if that goes through I'll teach you the spell." 

(The locket's not a Horcrux and hasn't been since about a week after they found out about Hermione; Harry was only holding off on it because he thought Voldemort didn't know they were after him yet. He is not actually sure if Voldemione knows this but he's sure not going to bring it up. 

"Agreed." 

Then he'll go fetch it. (It's exactly as secure as it was before; why not, when they'd already built the wards? Best case scenario she spends down a bunch of resources stealing it and gets approximately nothing for the effort.) 

While he's downstairs he sends Patronus messages to Remus and to Harry. 

When he returns she's still standing there in the exact same spot, teeth gritted, muttering something arcane that's probably holding the book together. It's turning unsettling shades of mottled glittering orange-green. 

The glowing white dog that is supposed to be delivering a message to Harry is walking in circles around her, sniffing at the book. 

Hate that, thanks. He dismisses it, frowning balefully. 

"What'd you do to them?" he asks, because there's a chance she'll say something that contains nonzero information even if it is a lie. 

"Harry tried to talk down my basilisk, because he is very stupid. Obnoxious firebird in five and it's not my fault if she lights you on fire." 

"It is incredibly your fault if she lights me on fire, you horrid cockroach." 

Harry did tell him about the basilisk but Sirius assumed, apparently incorrectly, that Harry was not planning to try to solo that on a random Tuesday in March. What the fuck, Harry. Sirius can't even be that mad at him because he too has absolutely made adventuring decisions at least this stupid but still. 

Yeah, yeah. How dare he defile the natural order or whatever.

Bargaining chip number one: no longer trapped in a book. 

KRRRRRRRRRRRRAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAWWWWK 

... 

...

Wow that sure is a tense standoff involving only two people who are both Unworthy(TM). Fawkes has no interest in this occurrence! Good-bye. 

Shields firebreak shiel.....  no? ... Okay.  

"Locket." 

Toss. 

Nailed it. Perfectly constructed soul-storage mechanism, currently empty of soul because the Gryffindors have done her work for her. She loves being the smartest and cleverest 

(no you're not) 

(shut up, granger) 

ahem, the smartest and cleverest wizard of several generations, thank you very much. 

"Lovely," she says, stringing it around her neck and tucking it under her shirt, next to the comically incongruous gold-and-scarlet tie. "Who's your Vowkeeper, then?" 

(Sirius can see a dozen spells go by under her fingertips as she puts away the book, too, no longer rattling; clearly she needed the locket somehow to stabilize it.) 

This is the Fucking Worst. 

"What happens if I say 'Dumbledore'," he wonders. 

"I laugh at you and leave and you never get your idiot back." 

Fuck, she actually wanted the empty locket more than the spell. This is a stupid game and he is bad at it. 

"Fair enough. Remus Lupin will be here any second anyway." 

Speak of the werewolf. 

"Huh. Nothing's on fire," he observes. "Honestly I'm impressed with both of you." 

He should be, this is so hard. 

Sirius, grumbling, explains the situation thus far while Voldemort smirks at him over his coffee table and waits patiently, his (her?) (their??) time-sensitive goal apparently achieved. It's probably a good thing he doesn't have to deal with this all the time like Harry does, he would actually explode. 

"... look I get it but you really shouldn't." 

Of course he shouldn't. Are you going to stop him, little werewolf? 

He doesn't want to but he's not sure he can face Dumbledore and say he stood there and let Sirius betray them. Again. Fool me once, etcetera. 

"...I could probably fix the trapped in a book thing," he says instead. 

"If we die and leave Harry in a book he stays there forever. If I kill you you just die a normal amount. I really really really do not want to but if I have to I will." 

Fuck he is not kidding even a little. 

Remus can take Sirius in a fair fight. It's not even that hard, comparatively; he's had the same blind spot since he was fourteen and learned a cool hex-sequencing trick that works on everyone who didn't watch him learn it.

Remus absolutely cannot take Sirius and Voldemort. 

"Fine." 

So he holds hands with the horrible cockroach and his prisoner, and waits for Remus to weave the Vow, and then he waits expectantly. 

She has a roughly hour-long detailed specification, memorized in contract format. The short version is that she hasn't lied to him (today (but is not committing to not doing so in the future)), really does have Harry Potter (the one with which he is familiar (still alive)) soul-trapped, and is in fact agreeing to release him (uninjured (et cetera) in exchange for the knowledge of the Fidelius Charm (defined as the spell with the following function...)(*). 

She recites it in its entirety without pause or consultation of notes, because why not show off, if you can. 

(*) Historical note: the only people currently able to teach the Fidelius Charm - a strict subset of the people who can cast it, because the knowledge is itself charmed - are Albus Dumbledore, Sirius Black, Andromeda Tonks, and the Baba Yaga (**). In most timelines it would have been possible to bribe this knowledge out of Arcturus IV as recently as last year (as Dumbledore in fact did many years prior), but Harry assassinated him before Voldemione could get to him. The people he personally taught the spell to thence became Secret-Keepers, which would have included a few more of his less-good-aligned grandchildren and grand-niblings if not for Andromeda having gotten away with Obliviating her sisters when she was fifteen.

(**) Additional historical note: This happened in the 50s. It's probably fine.  

He was honestly kind of hoping she was lying and they could instead fall back on the plan where he and Remus try to kill her, which would probably not work but at least it would feel less like shredding all that remains of his principles.

He agrees to the vow. 

It doesn't even hurt. It kind of seems like it should. 

It takes, on average - and this is an average of a distinctly non-average population, mind - six months to learn to cast the Fidelius Charm properly. This has very little to do with the quality of the teacher and quite a lot to do with being able to fit enough math into your brain as a single chunk, because if you try to pick up any of its component parts separately, the spell detects that you don't already know it, counts you as someone who is not supposed to know, and every piece individually forgets itself. 

Voldemort would have taken about six weeks to get it down, doing very little else. 

Hermione Granger gets it in four. 

Unfortunately (?), it turns out what happens if you both do and don't count as a legal target for a Fidelius Charm at the same time is quite a large explosion. 

Welcome to the void between space! There are no things! Please enjoy your infinite and also instantaneous stay. 

"Was that you?" 

"No. Yes? Maybe???" 

Ah, they are going back in time. Why is this a thing that keeps happening. 

"I am going to murder this horrible child's entire family so that she is never born." 

Probably she's going to stop being able to talk again when they reenter normal reality but that is future her's problem and present her has shared a brain with Tom Riddle for too long not to be tempted to gloat a little. "Not if you want me to teach you the spell you're not! Technically Sirius just finished fulfilling his part of the deal, you didn't specify which inhabitant of this body counts while you were neurotically making sure he couldn't cheat by teaching it to Nagini." 

Which she successfully avoided noticing until it was too late by focusing 110% of her attention on the shiny new spell, you see. 

Oh, he sees now why Harry was so upset about this girl probably dying, she's a delight. 

"Also he probably can't kill Lily and James until they have Harry since that is the only way to fulfill the vow to present Sirius with an alive Harry," she adds, thoughtfully, to Sirius. "You know. In case that's relevant to your strategic planning in a minute." 

"Incredibly relevant, thanks." 

Uuuuugghhhhh. "Someday I am going to Cruciate the lot of you for thirty consecutive years and you will regret all of your choices." 

"Believe it when I see it and not before, motherfucker, you don't scare me. ... And Granger, you are an absolute hero, I am so sorry about the horrible war he is about to wage with your hands, we will get you out of there some day, all right?" 

The funny thing is, yesterday she didn't think they'd ever manage it. Today she kind of thinks they might. "Thanks, Sirius. See you on the other side."