Disclaimer: I am not J K Rowling. I do not own Harry Potter. Or Lord Voldemort, Severus Snape, or any of the other characters in this story.

Note (revised/updated): The following is a one-shot set in an alternate universe. Sirius Black significantly deviates from canon Sirius Black. Other characters may also deviate from their canon counterparts.

Further Note: This one-shot commences with a couple of glimpses into events around Hallowe'en 1981, which plays out differently from canon given the different circumstances, looks at the immediate aftermath, and then skips on a number of months for the last scene or so.

(Author Notes have been slightly expanded and updated 11th July, 2012.)

"They made you their secret keeper?" the Dark Lord stared at his visitor, his voice a mix of amusement and awe.

"Ridiculous, I know." Sirius Black draped himself in a chair next to the sideboard and helped himself to a small glass of cherry brandy from the Dark Lord's personal supply. "It shows just how bloody stupid they all are, and how much they trust me."

"Apparently." Voldemort frowned. "If they do trust you this much, however, we need to preserve that trust to be able to continue to make use of it. Who else knows you're the secret keeper?"

"Right now, just you, me, James – who cast the fidelius – and Lily. A number of others know we were planning it – Dumbledore, Peter, and Remus – but so far I haven't been to see any of them to let them know where the Potters live, and James and Lily are relying on me to go let them know, since they're trying to stay safe and aren't going anywhere or contacting anyone."

Voldemort narrowed his eyes.

"What if – after the raid – you put out a story that at the last minute you and James switched keepers? Something like you were too obvious a target. So long as we silence whomever else the 'supposed' keeper was, it might keep any suspicions of treachery from you."

"Peter." Sirius sniggered. "His animagus form's a rat, which says all you need to know about his character." He paused, reflectively. "Yeah, that might work."

"It will mean disappointing Severus, of course, since I will not be able to leave Lily alive and free to contradict your story." Voldemort said. "He did request her life, as a favour, but…"

"Wait. Severus asked you what?"

"He begged me to spare the mudblood." Voldemort said.

"So do so." Sirius said. Voldemort raised an eyebrow and Sirius paused to sip his cherry brandy before going on to explain himself. "He had the most ridiculous crush on her at school, and apparently it's carried over into adulthood. You've had doubts about his loyalties at times, given how bloody good an occlumens he is. So long as you hold Lily, you have a little extra insurance against treachery on his part. And even if she's alive, who said anything about her being free? It wouldn't look unusual if she simply vanished, as it's not as if Death Eaters don't abduct the occasional victim for 'fun' during a raid."

"Is Severus really worth the inconveniencing myself with a long-term guest?" Voldemort wondered out loud, toying thoughtfully with his wand.

"He's a bloody brilliant potions brewer, and I reckon he's one of your best duellists of his age." Sirius gave a rueful grin. He raised his glass in semi-mocking toast to the absent Severus. "I spent enough years at Hogwarts fighting him. Believe me, we needed to take him four on one on some occasions."

"Hmm…." Voldemort said.

Severus had been summoned, and urgently.

He arrived in the Dark Lord's presence and found the Dark Lord looking amused and pleased and there, on the floor, lying bound and gagged, was Lily. There were just the Dark Lord, Lily, and Severus in the room.

"Harry Potter is dead." Voldemort said, watching Severus very closely. "The Potters' secret keeper, Peter Pettigrew, sold them out in exchange for his own miserable life. He has now left the country."

"My lord. I offer my congratulations, but I do not understand:" Severus said, struggling to maintain some kind of mental and emotional balance. "There was a rumour at Hogwarts that Sirius Black was going to be the Potters' secret keeper."

"There was a last-minute switch." Voldemort smiled. "Imagine my surprise when I captured Peter Pettigrew, intending to extract information from him about Sirius' whereabouts and doings, and discovered what they had done to try to fool me. And it would have worked too, at least for a time, had I not had the good fortune to stumble so upon the truth. Or at least," Voldemort purred, "that is the story which it serves me to be put about. James Potter is dead. Harry Potter is dead, the man who was the Potters' secret keeper betrayed them, and as you have requested, Severus, I spared Lily Potter, formerly Lily Evans, for you, and have her here as a very special guest, for your pleasure. After all, you are one of my most loyal and effective servants, are you not, and it is little bother to me for such a one to keep her here, only to be touched and done with as desired by you."

"Yes, my Lord." Severus said, his blood running cold. He bowed low. "Your generosity knows no bounds."

"Well, Severus. She is a widow now, after all, and as your lord and master, I would be happy to give you permission to take her and do with her as you please. You can even claim her as your wife, if you so wish. I would not object to you pairing with a muggle-born such as she, and given how long she has been in James Potter's clutches, you must be eager to at least renew acquaintances?"

Severus Snape was distraught, which was fortunate. It meant, when he confronted Dumbledore, that he had genuine distress to show him. It was distress about something other than what he wanted Dumbledore to believe him to be distressed about, but it was preferable that Dumbledore believe Severus was distressed about Lily disappearing rather than about her being the Dark Lord's 'personal guest'.

"You promised to keep them safe!" Severus accused the headmaster of Hogwarts. "And the Dark Lord found them all anyway."

"Severus. They were in hiding under the fidelius charm. James wanted one of his most trusted friends to be their secret keeper. They should have been safe." Dumbledore narrowed his eyes. "The Dark Lord must surely have been boasting of his triumph. How did he catch up with them?" the old wizard demanded.

"The Dark Lord says that the Potters switched secret keepers to Pettigrew at the last minute. He claims he captured Pettigrew, intending to torture him for information about Black's whereabouts, and purely by chance discovered the switch. He says Pettigrew traded the information for his life and liberty."

Dumbledore sagged, suddenly, looking very old.

"Unbelievably stupid and ingenious of them to switch at the last minute." he shook his head. "It could have deceived Voldemort for years, if he'd simply been looking for Black, instead of putting his hand straight on the man whom they had picked, at the first instance." Dumbledore rallied. "There were two possible children of prophecy, and clearly, now, I shall have to act as if it was the Longbottom boy, Neville, indicated all along. If it was not then we may well be in for some truly dark times. Severus:" Dumbledore paused. "Only the bodies of James and Harry were found in Godric's Hollow. I do not wish to give you false hope, but it is possible that Lily may have escaped somehow and be in hiding, perhaps injured, somewhere."

"Or it is possible that she may have been taken and tossed by the Dark Lord to one of his favourite pure-blood Death Eaters, such as Bellatrix's brother-in-law or Dolohov to amuse themselves with until they are quite done, and then a quick avada kedavra and dump her body somewhere." Severus said coldly. "Oh, I shall certainly keep both my ears open for any rumours of what happened to her, and I shall also be keeping my ears open for word of the one who got her in this mess. The Dark Lord said he let Pettigrew go, and he has since no doubt departed the country, but you had best keep any idiot who may have helped James make his stupid decision over the secret keeper clear of me, too."

After much analysis, Severus had concluded it was best to present to Dumbledore a front as if he believed Lily to be dead, and had no hope but for revenge on those responsible for what had happened to her. That would be a plausible reason for him to wish to appear to continue to act as a double-agent and to stay close to Dumbledore. Right now, it was essential that Severus stay as close to Dumbledore as possible, because that was what Voldemort wanted him to do, and Voldemort had Lily, the only thing left which mattered in this war.

Sirius Black had feigned due distress and outrage over what had happened in Godric's Hollow, and the headmaster, the old fool, had swallowed it as he had always swallowed Sirius' little stories. It had been the werewolf incident when Sirius had tried unsuccessfully to feed Severus Snape to Remus Lupin that had opened Sirius' eyes to the power he wielded over the old man, and the ability he had to manipulate and deceive him. From that point on he had had little but contempt for the old fool, though he had hidden it well, and had gradually drifted back to his Black roots, able, when he returned to the bosom of his family, to present himself as one who had successfully infiltrated the good graces of the great wizard Albus Dumbledore. Oh yes, he had earned his parents' respect that day, when he presented himself as the true student of Slytherin who had infiltrated the house of his enemies.

It was just as well Sirius had been back with his family and had presented himself (after concluding school) as a secret ally from House Black to Lord Voldemort, since it had put him in a position to head-off Regulus from becoming a Death Eater. Regulus wouldn't have lasted five minutes after taking the dark mark, before having a mental breakdown or something – and he would have been demeaning himself as a Black by even taking the mark (which for some crazy reason he'd had a desire to do) in the first place. Lesser witches and wizards or the truly desperate took the mark, not Blacks, and Sirius was genuinely fond of his little brother and wanted him safe and sound somewhere other than wizarding Britain in the middle of a war.

Regulus had been shipped off abroad to marry some foreign witch of suitable background, and to ensure that there would be Blacks to continue into the next generation if something should happen to Sirius in the highly enjoyable dangerous game of deceit and betrayal he was playing with Albus Dumbledore. Every time he walked into an Order of the Phoenix meeting, there was the risk of possible exposure and a desperate fight for his freedom, and Sirius Black revelled in the thrill of every moment of it. Voldemort wanted Severus closer to Dumbledore these days, in case Sirius ever had to make a break for it, and had instructed him to pretend to be a Death Eater having doubts after the deaths of the Potters and to give Dumbledore certain information. As far as Sirius could gather from Order of the Phoenix meetings Severus was doing that just fine, and indeed he had to wonder if Severus might have been engaged in a bit of double-dealing on his own account before Hallowe'en, not that that mattered now that Voldemort had Lily.

Sirius had been unsurprised, however, that apparently Severus Snape had fed Voldemort's 'official' view to Dumbledore on the switch of secret keepers. The moment Sirius had heard Severus had begged the Dark Lord to spare Lily, it was as if a light had flashed into life inside Sirius' head and he finally understood the man. Severus was so recklessly dangerous and effective because there was only the one thing which he truly cared about, and that was not even himself, but another person. Sirius was sure that if put on the spot, Severus would throw himself in front of a killing curse if it would keep Lily safe and happy. He had virtually sold his soul to two masters, to try and save her. And now, of course, he had her, thanks to Sirius, and didn't want to do anything which would jeopardise her, and so Sirius was certain that Severus would be a good little Death Eater and do whatever Voldemort wanted, so long as Lily was appropriately handled. Although that 'appropriate' handling did include Sirius occasionally dropping by Lily's current accommodation to engage in some mild taunting. She had to see that she had been betrayed and failed by Dumbledore – she had to see that Dumbledore couldn't possibly beat Voldemort when he couldn't even spot the king-size traitor in his own Order of the Phoenix – otherwise Sirius had a feeling Lily might try to talk Severus into doing something very silly. So long as Lily was uncertain, afraid, and not trying to do anything other than keep her own head down and survive, Sirius and Voldemort were golden as far as Severus went.

As a pair, the half-blood and the muggle-born made quite a charming couple. They were suited to one another in terms of class and blood-status. Whilst Sirius wasn't a completely fanatical blood-purist, he considered it inappropriate for pure-bloods to marry a muggle or muggle-born without exceedingly extenuating circumstances in terms of at least expected wealth, greater social status or political gain. It was what had made betraying James to his death so easy for Sirius – the man whom Sirius had once considered almost a brother had married so far beneath him, betraying the lineage and standing of the Potter family. The Evans family had been nothing, and their daughter was an unworthy match for a pure-blood. It was entirely appropriate that the union be forcibly dissolved, the unrepentant traitor to his position in the Wizarding world eliminated, and the mewling creature spawned of the union stamped out. Pretending to actually care about his godson for over a year had made Sirius almost sick at times, and it was still a source of private annoyance to him when someone such as Albus or Remus would go on about Harry and he had to take up once more the charade. At least Sirius didn't have to pretend any more around Lily.

Lily Riddle – or Potter or Evans as she had once been – had discovered, over the months that followed that awful Hallowe'en and her capture, that she had fallen through into a nightmarish world of deceits, treachery, and betrayal, where the only thing that she could be certain of was that apparently Severus Snape was fanatically devoted to her, and had always been. Devoted enough to have begged the Dark Lord to spare her life, which could have got him crucio'ed or worse, and yet which the Dark Lord had done.

Sirius had betrayed the Potter family, including his own godson. He had boasted to Lily of how he was the Dark Lord's 'right hand', his most valuable ally, and how Dumbledore hadn't the faintest idea what was going on. Sirius had betrayed the Potter family to the Dark Lord, and the Dark Lord had killed James and Harry, and stolen Lily away – the Dark Lord had branded her with the mark of a giant snake, which she sometimes imagined was alive and hissing in her head – but he had spared her.

Lily had discovered other things about Severus, such as that his patronus had used to be a doe, like hers, but it had recently metamorphosed, and was a dragon these days.

He was lying alongside her, in their bed in their quarters one evening when he sat up and suddenly clutched at the dark-mark on his arm, and uttered a string of curses. And went very pale.

"Something just happened to the Dark Lord, Lily." he said.

The Dark Lord, Lily knew, had been becoming impatient with the progress of the war, recently, and had summoned her one night and subjected her to legilimency to discover all that she knew about the Longbottoms. He had been starting to believe that he had erred in assuming poor dead Harry had been the child of prophecy.

Lily felt the tingle of the snake tattoo move uneasily over her skin, as if it were disturbed too by something that had just happened to the wizard who placed it.

She saw Severus press his mark, and frown.

She felt the child within her stir, as if it too were uneasy. Kept in a gilded cage with limited options, letting herself become pregnant with his child had seemed a good way to ensure Severus' continued support for her. She had become unpleasantly aware of just how Death Eaters treated some muggle-born witches they took alive before disposing of them – not that Severus had ever threatened her or treated her anything less than decently – and of the looks the Lestrange brother who wasn't married to Bellatrix had cast in her direction when she had bumped into him the once in the main council chamber of this place before an inner circle meeting. It was weird how Severus wouldn't call her a mudblood – or even use or tolerate the word from others in her presence, except from someone of equal or higher standing in the Death Eater pecking order. He genuinely seemed indifferent to blood-status these days, which puzzled Lily.

She saw Severus reach for his wand, and conjure his patronus, and send it off to Sirius. A secret Severus had acquired from the Order of the Phoenix, used to contact the man who had betrayed James and Harry and Lily to Voldemort. Lily had no idea which side Severus was on these days. She knew Severus certainly acted as if he was a completely loyal Death Eater, but then there was his deep and obvious care for herself. She'd never seen such a situation arise whilst she was here, but she sometimes wondered what he'd chose if it came to a decision between her and the Death Eaters? If there had been any hope at all for the war she might have tried to work on him, to get him to turn on his fellows, but with Sirius betraying the Order and Dumbledore day in, day out, it was probably only a matter of time before the Death Eaters won, and after the moment she'd tried to save Harry but been brushed contemptuously aside by Voldemort some of her Gryffindor courage had died inside her.

"Get something on." Severus said. He waved his wand and lit several of the room's candles. "Sirius will be coming soon, and this will be important."

Lily grumbled and got out of bed, and pulled on a green silk kimono.

Sirius Black was unmarked because he was not a Death Eater – the heir of the Black family served no man or woman who was not another Black. He was, if Lily understood correctly, an ally of the Dark Lord, and for all that she knew it was quite possible that other than the Dark Lord, she and Severus were the only two people alive privy to that little dirty secret of Sirius.

"Well, well." Sirius said, breezing in ten minutes later into the current Death Eater headquarters. "Here's a pretty state of things."

He helped himself to a glass of cherry brandy, and then eyed Severus and Lily.

"Something's happened to the Dark Lord." Severus said.

"There's rather a fuss at the moment, Severus, old boy." Sirius drawled. "You will of course be aware how he wasn't happy with the direction and pace of the war, and had started to wonder if Neville Longbottom was in fact the child of prophecy destined to be his bane, and that this evening he went after Neville Longbottom? Well it spectacularly backfired, word's spreading, and the majority of Death Eaters are losing their heads. Which leaves me with something of a problem, Severus, old boy, since you and Lily know that I was the secret keeper for the Potter fidelius charm. Your leader may have indulged you Severus in permitting you both to live, and I respected his decisions, but your leader is now gone, blown up by a backfiring killing-curse, if you can believe that the toddler Longbottom somehow did that to him – he's clearly going to be a powerful wizard, some day, performing accidental magic like that. I knew Frank and Alice were from good families and should have made for decent allies or servants, and Merlin knows, I encouraged your leader to spare them both if possible. Alice was out, thanks to my decoy efforts, but Frank and your leader seem to have disagreed to Frank's eternal cost, alas. But anyway, Severus, I diverge from the main point which is as to what to do about the pair of you?"

"Which is why I summoned you here." Severus replied. "I foresaw that we would need to resolve the situation as fast as possible. You are the Black heir, and Black to the bone, and I assume that even in the event I drew a wand on you, and we duelled, you would have taken steps to ensure that my killing you would achieve little benefit to me."

"Oh, you're sharp, Severus, there's no doubting that. We both understand, as your leader did, your greatest strength and weakness. Now, I can relocate the pair of you to rooms in Grimmauld Place, and ensure any charges against you, Severus, of being a Death Eater, disappear, when it is revealed that you have in fact been an accredited agent of the Black family, spying on the Dark Lord for them. The Wizengamot wouldn't dare to lay a finger on a man like that, unless he had been caught literally red-handed, with smoking wand, assassinating the Minister for Magic. Lily will stay hidden, our secret. Your leader did mark her personally, after all, with his serpent, and if he ever comes back – he seemed confident enough of his immortality after all – I wouldn't want it counted against me that I had harmed such a possession unless it had been purely on grounds of self-defence. And afterwards – well you can work as a private potions master for the Black family, Severus, on a generous retainer – until and unless your leader should show up again…"

Author Notes (updated):

This one originated as a 'What if Sirius had actually been in league with Voldemort?' idea. He's a Black and members of his family in canon seem to me to have at least been supportive of some of Voldemort's objectives and methods at times, even if I can imagine them looking down somewhat on this wizard who's come out of nowhere and calls himself 'Lord Voldemort' but can't produce a convincing halfway decent family tree...

I haven't quite worked out what the Sirius of this universe tells Dumbledore about his connection with the rest of the Black family, but it's probably something along the lines of he's only pretending to go back to them so that he can spy on them for Dumbledore. He's probably picked up a bit of occlumency, and Dumbledore trusts him enough that what he has mastered will suffice. Given what canon Sirius could get away with at school, it seems reasonable to me to assume Dumbledore has something of a blind-spot where Sirius Black is concerned which this Sirius tries to avoid doing anything hugely incriminating to move himself out of.

As far as the Death Eaters go, only Severus knows Sirius drops by regularly, and that only after Hallowe'en 1981, once Lily gets drawn into things, since he's going to find out from her who the Potter secret keeper was anyway.

Sirius convincing Voldemort to give the Longbottoms a chance (because they're pure-bloods of a good family) is ironically what finishes this version of Voldemort off when he goes after Neville, with parental sacrifice from Frank. That occurs at some point in 1982.

Sirius has in place some measures to hurt Lily a great deal if Voldemort isn't on the scene and Severus goes after him. (Things such as a will with directions to bounty hunters/assassins putting a price on her head.) Sirius is pretty sure he can't take Severus down in a fair fight, but he has what amounts to a loaded gun pointing at Lily to ensure Severus' continuing cooperation.

Severus doesn't ask Dumbledore for help rescuing Lily whilst Voldemort is still active because the consequences of a failed rescue attempt are catastrophic (Lily's death or worse). Dumbledore already messed up over her safety once, allowing Voldemort to capture her, and Severus isn't about to risk Dumbledore messing up yet again.

As regards Severus' patronus, it changed since under the circumstances he's had to become a lot more fiercely possessive and protective of Lily, and a dragon seemed more suited than the old soft romantic notion of the matching doe...

Lily's living snake brand/tattoo is Voldemort's last intentional horcrux, intended to mark his 'triumph' over the prophecy and child of prophecy, placed shortly after Hallowe'en 1981, when he was certain he'd won. (I used this idea for a horcrux before in the parody 'Harry Potter and the Universe too Far' - the idea was originally partially inspired by the 'mark of the Mara' in the Doctor Who stories Kinda and Snakedance.)

This story is a one-shot. Regarding what might happen afterwards, in the longer term, Sirius' own weak-spot of his younger brother (and possibly nephews and nieces) would probably eventually come back to bite him and result in his exposure and downfall. Severus might (if he could find a way to do so that would keep Lily safe) turn on Sirius at a crucial moment. In theory, Sirius is taking a huge risk by keeping both Severus and Lily around, but he enjoys the danger...