Sev padded through the corridors of Hogwarts. There was no need for concealment magics - the entire castle was deserted. Everybody was outside, whether they were part of the festivities or merely watching them.
Sev had spent enough time sequestered up in this little tower room that even had he not had half the memory he did, the route would have been permanently ingrained. He twisted the arm of the statue, slipped through the gap it stepped aside from, and ascended the stairs.
He smiled when he came to the door at the top. He and Dumbledore had played the most brazen bluff imaginable. He had spent months up here with the Foe-Glass, designing a magical lock for this door that would perfectly suit his needs. And until he'd completed his work, there had been no lock on the door. The Death Eater could have sauntered in at any time and taken the Foe-Glass, without anything to stop them or mark their passage.
But Sev had known they wouldn't. The hidden Death Eater had proved to be incredibly cautious; so cautious that they would never presume to test the glass's defences until they knew exactly what those defences were.
It had taken much of his ingenuity to work out a suitable interface for the lock. It hadn't bothered him to fiddle around with the raw Arithmancy of it, applying a little power here and a little there to get the right balance. But none of the staff, except perhaps for Fractalis, would be able to remember a code built of such equations. Other people's memories, he was aware, seemed to require something more tangible to latch onto.
So, largely for his own amusement, he had made the lock into a kind of little solar system. A collection of little stars and moons and planets hung in space, and their relative positions made up the code. It would be easy to replicate a given arrangement, but literally impossible to try and cycle through all the different combinations to find one that worked. Had he been the kind to share his ability to make puns with anyone, he would probably have named it the 'Universal Lock'.
In addition to simply permitting or refusing entry, the lock had another handy little hidden trick. Sev, never leaving anything to chance, drew his wand to test its operation one more time.
"Recyclius!"
Immediately, the little stars and planets began to 'orbit', moving around in the loops he had restricted them to. After a moment, they all stopped moving. With a click, the door popped open.
Satisfied, Snape peered briefly inside, and shut the door again. He pointed his wand at the 'sun' in the centre of the arrangement, and said "Randomius!" The planets slid around their orbits and came to random stops. He leaned his weight against the door to test it, and then turned and descended the stairs.
So far as the Death Eaters were concerned, what he had just done up here was switch off a spell to identify the people who passed through the lock. In fact he hadn't, because he'd already designed it so that that spell was there if anyone checked for it, but never activated.
There was no way to detect identity that was completely foolproof. These days, most Ministry-approved security systems had devised ways of seeing through Polyjuice Potion, but there were plenty more devious methods available to a talented trickster - and the Death Eater agent would certainly be that. Cautious or not, it took an incredible amount of skill to leave no clues that either Snape or Dumbledore could pick up.
Lily was waiting for him when he emerged from the buildings. "Where have you been?" she asked, eyebrows raised in curiosity.
"Bathroom break?" he said guilelessly. She snorted.
"Yeah, right." The two of them wandered away towards the edges of the Forbidden Forest, where they would be out of view.
"What's happening, Sev?" Lily asked him, playfulness dropped as reached the shelter of the trees.
"Aside from the festival?"
"Aside from that, yes."
Snape shrugged, and looked at her. "What do you think's happening?" he asked.
She didn't disappoint him. "You're setting a trap to catch the spy."
"Find the spy," he corrected. She shrugged and frowned.
"Same difference, surely?"
He shook his head. "I want to know who the spy is, not blow their cover."
"Why?" Then she answered her own question. "Because they'll send another. And you're better off with a spy you know about than one you don't."
"Do I need to be here at all?" he asked sardonically. She rolled her eyes at him.
"Okay. So you'll find out the identity of the spy. And what are you planning to do with it once you've got it?"
"Remember it?" he suggested.
She glared. "Is that what I meant?"
"I'm a creature of logic. You can't expect me to guess what's going on inside your head."
"You're a creature, all right. I meant, who are you going to tell?" She folded her arms. "'Cause that was the point, wasn't it? I mean, at some stage in this whole 'I'm a master spy for the side of light' game, you're actually gonna tell the side of light you're spying for them, right?"
Sev smiled internally. As it happened, he was well ahead of her right here. "And who would you suggest I go to?" he asked neutrally.
"Dumbledore," she said, without hesitation.
"The logical choice," he agreed.
"Exactly!" she seized on that. "Logic! That thing that you're so fond of. So if that's the logical thing to do, why don't you do it?"
"Because I've already done it," he said. And then smirked at the way her jaw dropped.
Lily got her facial expressions under control, after a moment. "You've already been to Dumbledore?"
"We spoke last year." And actually, Dumbledore had come to him, but that didn't really matter. Sev had been planning on pulling him into the loop sooner or later - probably, he had to admit to himself, much later. He knew that his mission would be useless if he never reported to everybody, but he'd wanted to delay as long as possible having to bring somebody else in. While he worked alone, he was completely untouchable. Introduce a contact, no matter how trustworthy, and everything became a whole lot less certain.
"And he... knows about your little plan?"
"Actually, it's our little plan," Snape corrected.
Lily blinked at him for a moment, then pulled away. "Wow," she said, apparently to the nearest tree or possibly to the world in general. She looked back at him. "Dumbledore knows what you're doing? He... what did he say?"
"That he wants me to keep doing it."
Her eyebrows shot up. "Really?"
Sev half shrugged. "Basically. I seem to remember there was something in there about trust, foolishness, and people needing illusions, but I tuned that bit out."
"Of course you did," Lily agreed wryly. "Why listen to the bit that's telling you how to be a human being?"
He shrugged again. "I seem to do just fine without it."
"Yes, but fine by whose standards?"
"By mine. Who else matters?"
"See, this right here is at the heart of the 'human being' thing. You need to work on that."
"I don't think I do," he said lightly. In the brief lull in the conversation he was suddenly aware of the sound of soft footfalls on the forest floor.
"Someone's coming," said Lily.
"Ten out of ten for observation. Not to mention obvious remarks," Sev told her quietly. There was no point trying to run off - they waited warily for the approaching figure to emerge from the trees.
It was Professor Malachite. He came to an abrupt stop when he saw the two of them, looking confused.
"What's going on here? Severus?"
It was Lily who answered him. "Snape's being an idiot, that's what's going on," she glowered.
"Me? It was you and your stupid boyfriend who-"
The instant fake argument had the desired effect. "Okay, that's enough, both of you." He frowned at them. "Why aren't you at the festival with the others?"
"We're supposed to be running messages for the teachers," Lily told him.
"Or we would be, if somebody knew where they were going," Snape said sharply.
"Hey, it was you who said Professor Parilia was-"
"No, if you recall it was actually you that-"
"Okay, okay!" Malachite shouted them down again. He looked up at the sky with a long-suffering sigh. "What idiot decided to team you two up on this, hmm?"
"Professor Dumbledore," they answered in chorus.
Malachite laughed, and shook his head. "Oh, Albus. Ever optimistic, I see." He frowned. "Well, no wonder I haven't been getting any messages. Have you seen any of the other staff around here?"
"Not since Mrs. Potter here decided we should take a scenic detour," growled Snape.
"Mrs. Potter?" Lily yelped. The thump she gave him in the arm certainly wasn't faked.
"Oh, why can't the two of you just shut up and get married? Although I suppose that leads to the possibility you might reproduce, which is certainly something we want to avoid at all costs-"
She thumped him again, and Malachite looked like he honestly couldn't decide whether to be angry or amused. "Lily, I know this is a special day, but do you think you could see clear to keeping your fists to yourself?"
"Well, can you make him keep his words to himself?"
"It's self defence!" he objected. "If I don't talk, you might never shut up! And then sooner or later I'd have to kill myself..."
"Okay, I have to say I'm not really seeing the downside here..."
"Will the two of you please just-?" Malachite suddenly froze, and for a moment it looked as if he was about to spout prophecy like Professor Alomancia.
"Professor?" asked Snape, after a second.
Malachite blinked his sharp grey eyes and focused back on Snape. "Huh? Oh, yes." He reconfigured his face into a stern frown. "Both of you, stop bickering and attend to your duties. Severus, you should know better; don't let her goad you into retaliating. Now, if you'll both excuse me, I have some things to attend to." He marched off rapidly in the direction of the school.
"Okay, is it just me or was he doing the whole possessed thing just then?"
"I don't know about possessed," Sev said dryly, "but yes, something suddenly caught his attention."
"Let's find out what." They followed the route the professor had taken. It wasn't difficult; he was moving at quite a lick.
"I like how he managed to get in the one anti-Gryffindor line even while he was being dragged away by the vitally important thing that grabbed him," Lily observed as they trotted after him.
"You should be kinder to him; he's an oppressed minority."
"Really?"
"Yes. A Slytherin who doesn't like not being liked."
Lily suddenly stopped dead, feigning complete and utter shock. "Ye gods! A Slytherin with... actual human feelings?"
"Stop the presses," Sev agreed dryly. He looked at her. "Do you mind? We're actually in the middle of something here."
"Oh, can you stop it with the logic for just a couple of seconds? You're really no fun to have adventures with."
"Well, that's something I'll certainly have to work on," he said sarcastically. Then it was his turn to stop abruptly as he noticed the flattened grass ahead of them.
"What?"
"Look at the ground."
Lily looked. "It's a dent." He rolled his eyes at her.
"It's a rut."
"Well, you say potato. There's a rut in the ground. So what?"
"I've seen marks like this before, out in the Forbidden Forest. But never this fresh before."
"So Hagrid's been rolling barrels of Butterbeer about. There is such a thing as overthinking, you know?"
"You can never think too much," he refuted. "On the other hand, it's easy to talk too much. Do you think you could possibly be quiet?"
No doubt Lily would have argued, had she not acknowledged the wisdom of hiding their presence from Malachite. They broke from the cover of trees just in time to see the door ahead of them fall closed.
"He's gone back in the school," Lily observed.
Snape rolled his eyes at her. "Are they paying you to be the narrator?"
"They're not paying me for any of this, actually."
"You're free to leave. In fact, please do."
She gave him a look. "Oh no. You're not getting rid of me that easily. Besides-" she reached inside her robe and yanked out the wadded up invisibility cloak. "You need me."
"You always carry that around with you?" he asked her.
"Only when I think you're up to something."
"All the time, then?"
"Yeah, pretty much." She shook out the cloak and stretched it over both their heads. "Let's go see what he's up to."
Sev would never have chanced trying to observe the theft under normal circumstances, but the invisibility cloak changed matters. As did the presence of Lily. It was probably easier to go along with what she wanted to do than get caught short because the two of them were standing around arguing it over. And if they were by some chance spotted, it wouldn't be too hard to make it appear that Lily had suspected something and he had followed her to make sure she didn't witness anything.
It wasn't the world's easiest thing to move surreptitiously when there were two of them under the cloak. There was enough room for them, barely, but if they didn't coordinate their movements to some degree they were liable to lose their cover without warning. At least Lily was light-footed.
With the restrictions imposed by the cloak, it took longer than it should have done get to the tower and begin to ascend the stairs. Lily was obviously itching to charge on ahead, but Sev's more cautious pace ruled the day. The last thing they needed was for one of them to trip or mis-step and make a noise.
They were about halfway up as a sudden almighty crash sounded above. Lily jumped, causing the cloak to spill to the ground. Sev quickly snatched it up and looked at her. "Let me take this."
She looked like she was going to object, but there wasn't time for that. "Remember that spell I used in the first year? The one that made Fennel not see me when I was standing in the shadows?"
"Vaguely." She made a half-correct stab at the pronunciation, and he corrected her.
"Use that. Go down to the bottom of the stairs, and wait to see if anyone comes down. Go!" he ordered, when she still hesitated.
She looked annoyed, but pulled away and started to hop lightly back down the steps. Sev threw the cloak back over himself, and continued to ascend at a faster pace.
Even as he scurried up the stairs, his mind was working overtime, trying to analyse the source of the resounding crash that they'd heard. It had sounded like somebody crashing into a piece of furniture, hard. The sound of somebody trying to break down the door? But if so, it couldn't be the Death Eater. Or any of the other teachers, because they all had the code.
He couldn't even be fully sure it was Malachite who was up there. They'd followed him out of the woods, but they hadn't seen for certain it was him who'd entered the building. And even if it had been, he might have been going somewhere else...
Too many variables. There were too many variables, and he didn't like that at all. He slowed to a complete halt ten steps from the top, and listened intently for a moment. He heard another sound, a smaller thump, like something striking stone, and a strange kind of hissing.
What was going on up there?