The strangest part, Lily decides, is the fact that the grounds of Hogwarts haven't changed at all. The quiddich pitch is still standing with its hoops; proud little pureblood boys and girls could be zipping around on brooms in the morning. She's under the cloak and the moonlight falls right through her, straight to where her boots make their soft imprint on the grass.

Obliterating their footsteps behind them in the snow where it remains was tedious. The construct is silent, Disillusioned and padding before her under its own cloak, and beneath that a Death Eater mask and robes that aren't Severus' own. That's its own idea-if discovered, a ruse that could buy them time. But what it can do is scale the outermost wall as if nothing is there, even though Lily can feel the heat radiating off of the wards that go straight up from the wall, so hot it's like standing in front of a fireplace. It's a beast, a thing, constructs are too obscure to guard against and any general ward that could keep it out would be terribly strong and fiendishly difficult. That part was easy enough; birds and beasts have always been able to traverse the wards freely, and while there is a resonant magic at the core of the thing in sympathy with Severus, it isn't really human-or if it is, the human part of it is still in Cokeworth, probably staring at the ceiling of his bedroom or working in the lab or, heaven knows, fretting at the hem of his bathrobe. Lily isn't fool enough to think he'll be sleeping during their expedition no matter how late the hour.

The chain locking the side gate succumbs easily to a pair of plain muggle bolt-cutters, the door beyond the pitch takes an alohamora simple as you please, and Lily is almost aghast at how easy this is right up until they enter the first floor and she hears it; a horrible sound, like the rasp of steel against stone, but also like something else entirely-almost a voice.

Lily has to risk it. "Do you hear that? Like-a sword, being sharpened, almost."

There's a moment of silence, the rustle of robes beside her, as it moves to try to catch the sound. "No," it whispers, shockingly close to her ear. "If you had allowed the time to do further research, it's been years since I've been in the castle and the changes-"

"Time we don't have. It's stopped anyway. Come on."

They've been loaded down with all the tools Severus could lay hands on inside of twenty-four hours; a sneakoscope letting off a steady low pulsation in her pocket (the whole castle is Unplottable, after all, but it should get stronger if they come across anything really interesting); a variety of Dark Detectors of varying sensitivity and specificity (she's already had to turn three of them off, which is worrying but not entirely unexpected); and a bandolier loaded with healing potions, molotov cocktails, anything and everything Severus was sending to the front and could pilfer (he had been frustrated with its capacity and threatened to charm it to carry more until she pointed out it would be useless if she couldn't find anything on it).

The first stop is an initial cursory lap around the first floor, hoping for some kind of direction from the various sensors. One is in Lily's hand, a smooth and shiny black stone, concave on one side to fit her thumb, her favorite and the most useful one. It was modified for Albania to go off in a very peculiar way-a heat and, when close enough to truly powerful Dark magic like a horcrux, it gives horrible, sickening thud-thud of vibration, like a living heart's beating in her palm. When they had made it, finally gotten it to respond only to the horcruxes in the books and no other Dark artifacts, it had heated in Lily's palm so quickly it left blisters, beating a wild tattoo that skittered it across the table. Lily's own spells managed to fix it, managed to shield it from whatever bizarre error in her spellwok had set it off, and from then on it had only worked in her palm, so: her favorite. Privately, it reminded her of Severus. They had decided it would have to be enough.

-It isn't. There's nothing. Every step seems to suck the warmth from her palm until her fingers are stiff and cold curled around the detector and it doesn't so much as twitch.

"Dungeons," she whispers to the nothing next to her, but then she freezes. "There it is again. That sound."

"It sounds like water in the pipes," it mutters dismissively.

"It's not, it's someone talking-"

The construct cuts her off, pulling her bodily into an alcove. It's invisible but its familiar palm is pressed to her mouth, and she hears what it has, what she's missed: footsteps. A thrill of fear runs through her.

The owner, some heavily-cloaked jackbooted caretaker-an elder, possibly slightly disgraced member of the Rowle family, if the resemblance and a faint recollection is anything to go on-trundles past them without a glance, grumbling about that damn door and bloody kids. There must've been something they tripped when they came in, something silent that woke this man.

The caretaker is still woozy with sleep, and his wand falls out of his fingertips just beyond their hiding place. It rolls away from him, toward them, stopping just short of Lily's boot. Muttering, the man curses, turns and looks-he must sense them somehow, he's looking right at them. Lily stops breathing and the construct's hand tightens across her mouth. He takes one shuffling step and then another, fingertips reaching. Lily lifts her wand. If he has detected them, it would be safest-it would be best-to end it now, before he has his wand. It would be best to end him now.

But she doesn't want to kill this old man in cold blood, not even if he's going to raise the alarm, not even in self-defense; not wanting to do a thing and not doing it are on entirely separate planets but she doesn't want to-

He's reaching, his fingertips are on it, and then his wand is in his hand again and he's gone, down the hall, muttering about the children in this castle and how they'd all been raised in a barn.

When the footsteps fade and Lily's heartbeat returns from its manic pace to something normal, the palm lifts away. This is what Flamel was trying to tell her, and it chills her to the bone. That this could demand of her the death of not just Marked men and women, not just open adversaries, but innocents-people caught in the crossfire. Flamel had warned her, and she hadn't listened.

There's no time to worry over this particular question of morality. "Dungeons," she whispers again, and the construct's footsteps pad away toward the stairs to guide her. Tom Riddle was in Slytherin; it would make sense for him to stow a piece of his soul in the dungeons there.

Severus knows the dungeons better than Lily ever will, and this construct guides her unerringly through as much as it can; secret passageways and hidden corridors, past the Slytherin common room where Lily remembers the end of one particularly nasty argument with a queer jolt of nostalgia, classroom after classroom, each door charmed to ease open silently.

It's maddening. It's exhausting, nothing like searching the house. In her pockets, two more Dark Detectors have to be turned off, leaving only the one in her hand; the Slytherins have been at work and there are doors they cannot open silently. She could be walking past the horcrux a thousand times over and never know.

As they explore, it becomes distressingly clear that it would be very, very easy for all of this to fail and very few ways to succeed. The secondary plan-kidnap a house-elf and let Severus work whatever cruelty he can upon it for information-looks horribly likely. They won't leave empty-handed. While she had known this for a theoretical, it is slightly more horrible to contemplate when facing down the actuality. Smaller evils weighed out on a grand scale against bigger evils until the balance is found and hope no one comes out too tarnished for the weighing.

They dodge Slughorn with greater ease as he sleepily makes patrol around the corridors; he never was terribly perceptive. They press themselves to a wall to allow the Bloody Baron to pass, even as Lily considers waylaying him to ask questions about Helena Ravenclaw. But that ghost was taught by Salazar Slytherin himself, and it would be too optimistic to believe he'd help a silly little muggle-born sneaking around the school.

The only luck is that no alarm goes off. No hue and cry. One wandering cat who can clearly smell them catches a sleeping hex and curls up on the spot. But there's also no horcrux, nothing even close to it, nothing even worth inspecting.

It's almost a half-hour before the sound comes again, clearer than it ever has been, while they are sneaking past a snake-covered tapestry hanging in the wall-it sounds as if it's coming from within the tapestry, within the walls, and again, it's almost speech she can understand, almost. She lays one hand on the wall through the tapestry, then another, then presses her ear to it. And the part of it that sounds like a voice resolves itself, finally, into words:

"I smell a mudblood," it says, so close it might have been whispering in her ear.

Lily leaps away from the wall so violently that it sends her crashing into the opposite. But nothing comes for her, no wand, no green light, not even a doxy. Her head snaps left, then right, then straight ahead toward the tapestry. "Did you hear that?"

The construct makes an aborted, nasal sound-it's heard something. The sound continues, and failing any other plan, she creeps back toward it. Whatever this is, it wasn't here when she was in school, or-if the construct's silence is anything to go on-when Severus was here more recently.

Her fingertip traces one of the snakes on the tapestry. "Hello?" she whispers.

The blade-against metal sound increases to an almost jaw-aching intensity, and it says in that same voice: "Master?"

Very quickly-very, very quickly-Lily makes several deductions and one decision.

First, whatever this thing is, it can tell her blood status by smell and it can track her, and that is a problem. No one uses that slur these days without violent intent.

Second, it can both understand speech and speak, and can be confused by her. It can't identify her as the muggle-born it senses-yet-but master is quite a lot to work with in the balance of things.

Third, it is in the walls somehow. Whatever it is-ghost, creature, malevolent twisted spirit of the castle itself-it can trespass where they cannot. It must be huge to making the surrounding and encompassing noise it is capable of, or powerfully magical, or both, and Lily isn't looking forward to meeting it. Keeping it in the walls seems both dead useful and more likely to keep them both safe.

It resolves itself into an idea that becomes a decision, one she makes instantaneously: if some presumably-malevolent force creeping throughout the walls is sensitive enough to smell blood-status through walls, then it is likely sensitive enough to know where a horcrux is in the castle. And there's only one person she can think of this thing would be calling master.

The silver eyes of the serpents in the tapestry glitter at her like they're alive. "Yes, I am your master. I hid something here a very long time ago. Something that smells like me. I need your help to find it again."

For a wild second, the rasping sound grows even louder, stranger, new, and she wonders if it's all gone wrong already, lost her gamble, overstepped finally beyond the bounds of what can be recovered from-

It says, "Follow."

The sound moves away down the hall and moves to dart after it, and then halts, noticing a lack of fellow footsteps behind her. "What are you waiting for?" she hisses. "Come on."

It falls in line, soft padding steps along her own. Severus is taller than her by a head and so is the thing, so it has to duck to whisper to her. "What did you do?"

"I talked to it. Didn't you hear me talking to it? It can talk, I talked back, it called me master and I took a gamble. Was there some kind of jinx that stopped you hearing all that?"

"No," it says shortly. "I wouldn't describe what you were doing as talking."

"What is that supposed to mean?"

It doesn't answer, and she can't interrogate it and also follow the sound as silently as possible, which has picked up speed and is ascending stairs upon stairs.

By the time they reach the seventh floor, she's almost given up on stealth; she thought she saw another pet cat run away from the sound so she supposes whatever this thing is, it scares even the residents. She suspects it should. She suspects it should scare her more, too.

Where it stopped, it seems there is nothing; no doors, no statues, just an ugly tapestry with dancing trolls and a darkened hall.

"What now?" Lily asks.

The voice in the wall doesn't answer, just maintains its alien rattle.

"You are leading us," the construct mutters, the implication being and you've brought us here, so figure it out.

"I'm not leading us, the thing in the walls is," she whispers, pressing the heel of her hand to her forehead. She paces away, almost to the tapestry, thinking furiously and trying to ignore the noise, and then back, and then away, and then back-

"Lily-"

It hasn't said her name before, she realizes, and the way it does it arresting. She spins on her heel toward it, and there's a door where, moments ago, there was only a wall.

And the cold pebble in her hand suddenly heats in her fingers.

There's no room for hesitation. If the door can appear, it can disappear, and the detector doesn't lie, not like this. She opens it, flings her body through, into whatever lies beyond.

The construct follows only after a moment's hesitation, shutting the door as quietly and as quickly as possible behind them.

-What is beyond is chaos. It is not the safe, curated, trapped place she had anticipated, not the sanctum sanctorum or even a pit filled with spikes or snakes or spiders or ghosts. Thin moonlight filters through tall windows that stretch from the floor to the distant ceiling, illuminating crooked towers of-it steals Lily's breath to realize it-junk. It's dumping ground: ruined potions, broken furniture, books doused in pumpkin juice and covered in graffiti stacked far above her head, half-melted cauldrons, joke items she recognizes from her time at Hogwarts and older things that must be more of the same, broomsticks and cloaks and student's robes and weapons and every imaginable illicit item ever smuggled into a school by idiot children who didn't understand the danger of magic.

This isn't the Dark Lord's hiding place. This is just a hiding place. If it's his, too, then he's a greater fool than even Lily had thought.

She looks around the maze to try and get her bearings, but there's no time to dwell or marvel; the sneakoscope's vibration in her pocket is so violent she has trouble removing it and casting a freezing charm to stop it. They are not unheard; something rustles in the distance, and a shadow passes across a window. The black pebble she's been carrying this whole time is still warm in her palm, but isn't doing anything more, and Lily holds her breath, holds it up in her fingertips, giving it a small shake.

And then it lets out one sluggish pulse.

"Turns out he's an idiot," Lily says breathlessly into the air, pushing the hood of the invisibility cloak back onto her shoulders.

The construct is making sure the door is secure before it turns. "What's happened?" There's a sense of warmth, a solid body invisibly standing before her. "What does it detect?"

Lily flicks the front of the cloak, exposing her forearm and the Dark Dectector in her palm. "It's detecting something."

A fingertip's worth of pressure comes on the stone in her palm, testing the heat there. "Then we follow it."

Far away, deep in the room, there is a half-human, half-monstrous scream, and they both turn to look into the dim rest of the room.

"Lead the way." There's a movement of air in front of her face-a hand that fluttered past her cheek, almost reaching to touch her but then retreating, as if remembering who and what it is.

Lily tugs the hood back over her face and starts to withdraw the arm back beneath her cloak, and then stops and extends it again. "Take my hand. We don't want to get separated."

The palm that meets her and the fingers that entwine are cool and familiar and hold tight. It might not really be him, but it's close enough.

Hand in hand, they begin to slide along the front wall where the door is to get their bearings. The heat ebbs and flows in the detector until she finds a place to enter the labyrinth where she thinks it's warmest. They step over brooms, over bones, over weapons at the end of a long dark trail of stain rendered black in the darkness. Past a stuffed troll, past a cabinet, where the pulse picks up and Lily's heart mirrors it, past another cupboard with blistered lacquer that stinks the way only rotting flesh can. As they move deeper, the beating ebbs, and Lily moves backward until she's back at the cabinet once more and the detector is fluttering in her palm like the beating of a rabbit's heart in the mouth of a wolf. She drops the thing's hand as she turns.

-And there it is. No tricks. No traps. No guile. It's just like the drawings, sitting among the trash. There's a bust of a wizard sitting on a stack of pages torn from a book, there's a wig and enchanted playing cards whose faces keep half-changing between suits-clubs to hearts, hearts to clubs-and atop the small illustrated crowns there is a tiara adorned with blue gems.

"This is-don't touch it, Severus!" But it's already in its hand-floating midair-and she doesn't even have time to correct herself calling it by his name. She digs in a pocket and pulls out a handkerchief and extends it past the cloak. "Use this, at least."

"It's inert. It must be placed on the head to effect its curse," it says, a note of wonder in its voice, wrapping the diadem in the handkerchief. There's a breath's hesitation, and then: "Have you always been a parselmouth and I just never knew?"

"A-what?"

It's fading into sight; just an outline, the profile of nose and long hair coming into focus. The Disillusionment must be wearing off. It had been due; she'll have to recast it before whatever is screaming comes for them, but that is less interesting than what it's saying just now. "I know what it is, the thing that guided us here. I don't know if it can get in here."

The definition of parselmouth comes back to her. "You think it's a snake?"

The scream that echoed around the chamber when they began their search echoes again. The familiar face looks up, around, and then back to her. "Worse. A basilisk. I thought I had-he, the real one-"

"Spit it out," she snaps, impatient.

The eyes turn on her, and she can see in them a rising worry. It speaks quickly. "Slytherin's chamber of secrets contained a monster to enforce blood-purity. Everyone thought it was a myth until the Dark Lord unleashed the monster on the school. It killed any muggle-borns that tried to return to Hogwarts after the siege. No one was completely sure what it was, not when it killed so quickly and with such stealth, but it fits."

Her heart leaps into her throat. They aren't prepared for a basilisk. "How did you know? It's not as if you've seen it. You'd be dead."

The increasingly visible face looks momentarily furious. "You hissed at the wall, Lily, and it hissed back. It fits." Its face crumples slightly. "You will have to tell the other this detail. My guesses will not be as good as his about what this means."

She skips past why don't you tell him yourself and goes straight for the red meat. "So what does it mean? And you're going solid again, let me-"

The scream they heard earlier comes once more, this time from much closer, and both of their heads snap round to look for it.

"We are out of time," it says, its face hard, like it's just made a decision and isn't telling her what the decision is yet. Lily doesn't like that look, doesn't trust it, but it knows well enough to hustle her along before she can protest. "We must flee. Now."

"You're not-we could hold them-"

"Now, Lily!" it snarls, and snatches up her wrist to pull her behind.

Severus' legs are longer than her own and so are the construct's, so Lily is breathless and half-dragged behind it as it runs unerringly back down the path they came, leaping broken broomstick pieces and sending broken wands rolling behind them.

"It just looked like an oversized doxy," Lily calls. "We can handle one man-sized doxy. There's no need-"

"Doxies never give chase alone," he replies, neither looking at her nor slowing. "I do not fancy meeting a flock of oversized, hungry doxies with one wand and a cursed object to protect between us." One long arm darts out, tugs on a single tattered volume from a stack, sending the tower topping behind them to block the path. Lily follows suit, kicking at a wardrobe with twisted doors leaning crooked against a stack of armor, and the heap is sent rolling across floor with an almighty crash.

Around another corner and there it is: the door, just a few dozen paces away, a distance covered in moments. From beyond the door, they can hear it: the rasp of the scales on stone. Knowing what it is, now, it's unmistakable. And from behind, it's more than one, it has to be a flock by the sound-

Lily presses her ear to the door and hears a hissing tirade so violent-kill rip tear-she starts back from the door. "The snake is out for blood."

The thing turns, finally, pressing the kerchief-wrapped diadem into her hand in place of its palm. "I'm going to open this door. You stand at the side and run as soon as the basilisk comes through."

Lily looks behind them, and the monsters that have been chasing them finally emerge from the heap of junk, prowling, eyes alight; blue-skinned and taller than Lily, stunted gossamer wings preventing true flight but buzzing just the same, and the teeth-precious as a pomeranian's on a pest-are sharp and big enough to rip her throat out and dripping with slaver.

She tucks the diadem away and blasts monstrous doxy away over a stack of broken chairs, another into the air, but two move in to replace each one she blasts. The thing that stands before her is disposable, and she is being absurd, but she hesitates. "What about you?"

"Don't," it says, groping for her shoulder in thin air and, finding it, giving her shoulder a little shove into place against the wall as if securing her.

More of the monsters fall to more blasting hexes and even invisible they've targeted her, prowling forward, and there are too damn many. They'll be on them-both of them-if they don't act, and Lily can't think of any other way to escape either of the threats closing in on each side of the door.

It's now or never. The thing with Severus' face closes its eyes, hand on the doorknob, and pulls-

The rasping hiss of the great horrible snake is upon them both as the door swings wide. It makes a horrible sound-a sound that is both a slur and a scream and a hiss all at once-and the serpent strikes.

The construct doesn't flinch, doesn't move; it can't see it coming but it must hear, it must, and it stands there brave and stiff as a flagpole. The fangs catch it on the neck, and blood sprays in an arc up and back, splattering her face. A wardrobe explodes as the weight of a centuries-old monster whips through it, and the door flies through the air to explode at her feet. The scream the emerges from the construct is too familiar, too much, too horrible to contemplate, and then suddenly silenced as the beast opens its mouth wider-horribly wider-and swallows.

The snake's coils pile inward and inward, endless, enormous, the other threat scattering to the rafters like so many wind-up dolls. The last of the tail filling the doorway thrashes past her and strikes her bodily, crushing her momentarily to the wall; the breath goes out of her and there is an almighty and sickening crack from somewhere in her body where the diadem's hard gems have crushed a rib.

But the cloak is still on her, and it doesn't have her scent yet, and there is nothing else she can do. Lily does what it's told her, for once. Limping, hand pressed to her side, she darts through the open door, past the last of the basilisk's tail, and she runs and runs and runs.