Sev silently trod the last two steps, and with one last check that the cloak was covering him, peered around the doorway. What he saw, even he could probably have been forgiven for failing to predict.

The hooded figure directly across from him sparked immediate recognition; it was undoubtedly the same person he'd seen with Malfoy three years ago. That had been his only sure sighting of the hidden Death Eater, and he'd been sure to burn it into his memory. However, in an unpredictable turn of events, this sighting wasn't the most interesting thing in the room.

That honour would almost certainly have to go to the enormous snake with which the Death Eater was tussling.

The snake was as thick around as a human torso, perhaps thicker, and at least twelve feet long. It was the shimmering blue white of solid ice, with a pattern of interlocking shield-shaped scales.

Well, Sev reflected, at least the origin of those mysterious ruts in the grass was explained. Such a creature might be able to move silently, but it certainly couldn't do it without leaving its mark.

Under ordinary conditions the enormous snake should have had no trouble coiling around and crushing its opponent, but the Death Eater was backed into a corner and it was too big to squeeze in and get at the hooded figure.

The snake, in a display of non-reptilian intelligence, seemed to realise that, and pulled back to hiss. Its fangs were as long as any of Sev's fingers, and they dripped venom.

For a moment, snake and Death Eater regarded each other. Or so it seemed, although Sev could see nothing of the face shadowed beneath the dark hood. Then, as one, both struck.

The snake's jaws snapped, but it was the Death Eater's hidden dagger that found its mark. The creature suddenly let out an incredible screech and reared backwards. The hooded figure deftly ducked away from it and sprinted out of the tower room, hammering down the stairs so quickly that it was pure luck the invisible Snape wasn't crashed into.

Sev saw the hilt of a dagger protruding from the snake's underbelly. It thrashed, trying to bite at the handle but unable to get a grip to tug it free.

Sev hesitated for a long moment. He had a hunch about this creature... but if he was wrong, and it was just a wild animal, approaching it could well be his last move.

Still, sometimes you had to just follow your instincts. Sometimes, all logic could tell you was that if you waited around to be one hundred percent sure, it would be too late to do anything.

In a decisive moment, Sev tugged off the invisibility cloak and stepped into the room. "Stop," he said, as commandingly as he was able.

The snake abruptly stopped thrashing. It slowly turned its head to regard him with grey, cold, unblinking eyes.

Had it been an animal, he could have said something like "Stay," but he didn't think it was just that, and so he said nothing. Instead, he very slowly stepped towards the huge creature. It reared up to give him more room to get at the knife-handle.

Sev gave it a brief nod of acknowledgement, and grabbed the knife in both hands to yank it free. It took nearly all the effort his muscles could provide, and the snake let out another hissing shriek. Sev didn't panic or run back, understanding that it was only pain that made it screech.

The blade of the knife was dripping with a thick, dark substance that hissed against the metal. He placed it down on the floor in the corner, careful not to touch the blade.

Then he looked back up at the snake, which was watching him. "I know what you are," he said to it, looking it in the eye.

There was a sudden clatter of footsteps, and he looked around to see Lily come haring up the stairs. She skidded to an abrupt halt as she saw him and the snake. "Holy-"

She bit off whatever swear-word she might have been contemplating as Professor Dumbledore emerged from the stairwell behind her. Despite the fact that the headmaster was considerably more than seventeen years old himself, he didn't seem remotely out of breath.

"Professor," said Sev, with a nod.

"Severus," said Dumbledore, just as calmly. Lily gaped at the pair of them, and the headmaster's eyes twinkled mischievously. Sev decided it fell to him to address the fourth member of their little gathering.

"You may as well change back now, you know," he told the serpent.

Lily let out a little gasp of surprise as the blue-white snake abruptly melted into the form of a human man. He grunted and dropped to his knees, clutching at his side with a grimace of pain.

"Professor!" exclaimed Lily.

Malachite looked up at Snape. "Clever boy," he wheezed with difficulty.

"Naturally." Sev assisted Dumbledore as they both helped Malachite to his feet. The headmaster patted his shoulder with some concern.

"Carnus, are you hurt?" he asked worriedly.

The Defence Against the Dark Arts professor pulled aside his robes and lifted the undershirt to look at his wound. It made a jagged line against his deathly pale skin, but it had already stopped bleeding.

"It's almost healed," he said, dropping the shirt back down and dismissing it. "It takes more than a knife wound to kill one of my kind."

Lily had recovered a little, although her jaw was still dropped. "You're a Naga!" she realised.

"I'm that, yes," Malachite agreed, with a small smile. "Full marks on identification of magical creatures, there."

Sev nodded slowly, a lot of things abruptly clicking into place. He had known Malachite to be a Parselmouth, capable of talking in the snake tongue, but that was an ability that had long been intertwined with the more ancient bloodlines of house Slytherin. He had read of the Naga, but certainly hadn't had any reason to expect that he might meet one.

Ironically enough, it had been in the private library Malachite had granted him access to that he'd found the few details of the Naga people that were available. An ancient race of India, they were part-human, part-snake, and some of them had the ability to shift between the two forms. As soon as he had seen that over-large serpent fighting the Death Eater and acting with intelligence, the idea had clicked into place. The moment it had, Malachite had been the only possible suspect.

"Who stabbed you?" asked Lily warily, perhaps wondering if it had been Snape, and if so, whose side she ought to be on.

"Our very own hidden Death Eater, I should imagine," said Dumbledore quietly.

"Yes." Malachite looked frustrated. "I thought there might be an attempt to steal the glass whilst everyone was distracted, so I slipped away and put a ward on the tower door to alert me if anyone went up. I caught them in the act, but I was too late. They got the glass, and I didn't even see the person's face."

Dumbledore nodded, but didn't look too worried. "Calm yourself, Carnus, all is not lost. We were already resigned to losing that particular Foe-Glass, and it will avail our enemy little."

Malachite frowned in puzzlement for a moment, then realisation dawned. "It was bait?" he demanded.

Sev nodded. "I hope you realise you nearly ruined my plan," he admonished dryly.

"Your plan?" Malachite looked at him, and then laughed quietly. "Of course, I should have known you'd be up to your eyebrows in this." He turned back to Dumbledore. "Albus, you surely could have explained-" He broke off, and looked resigned. "You suspected me?" he surmised.

"I suspected everybody," said Snape dryly.

"He's like that," nodded Lily.

Malachite looked back at her. "And how did you come to be mixed up in this, young lady?"

"Wrong place, wrong time," she said, with a quick shrug. Malachite raised an eyebrow.

"By design, no doubt." He looked at Sev and Dumbledore. "So this was all a trap of yours? Well, I fear the mouse has sprung it, and escaped with the cheese." He nodded his head towards the open door.

"Not quite," Sev said enigmatically, and he moved to examine his magical lock. As he had suspected, the Death Eater had taken the time to scramble the positions after they'd opened it; well, he had his built in features to deal with that. He drew his wand. "Recyclius!"

"It's no good fiddling with that, Severus," Malachite told him, shaking his head. "They'll have used concealment magics to hide their identity, that's a given."

"I'm not looking for that," Sev said, not looking up from the little planets as they rotated through their 'orbits'.

Malachite frowned, and he and the others came closer to watch. "Then what are you doing?"

"Looking at the last code that was put in."

"But what good will that do you? You already know how to get in. And anyway," he gestured at the magical lock as all the pieces came to a stop, "that's not even the code!"

Sev looked up at him, and smiled thinly. "It's not the code I gave you," he corrected.

Malachite looked confused, and then surprised, and Lily began to grin. Dumbledore let out a hearty chuckle. "Our Severus is quite the schemer, Carnus. You should know that by now."

"I should, shouldn't I?" agreed Malachite wryly. "Very well, Severus - you've proved yourself a master tactician. If you gave everybody a different code, then whose code was used?"

They all leaned closer, as Sev once again stared closely at the arrangement of the pieces. He had recognised the pattern as soon as it had first clicked into place, but it was in his nature to check, and check, and check again.

"Professor Vitae," he said abruptly, straightening up.

Lily stared at him, mouth agape. Dumbledore let out a melancholy sigh, looking up at the ceiling, and Malachite began to slowly nod.

"Ah, Ellida," said Dumbledore, sadly shaking his head. "I had hoped I was wrong about her."

"What? No, that can't be-" Lily looked from him to Snape in disbelief. "But she's a-"

"Gryffindor?" Malachite laughed, a sound without much humour. "Ah yes. Who would suspect the mighty Gryffindors, when we, the Slytherins, the snakes, are here to embody all that is wrong with the universe?" There was a bitter twist in his accent on 'snakes', and now they all knew why.

"No, but she, I mean-" Lily shook her head. "She's a Death Eater...?"

"I know it seems hard to believe," said Dumbledore. "I've had my suspicions for some years now, but I had been praying I was wrong. She was always such a bright child..."

"Not to mention a vicious, spiteful one," said Malachite darkly. "Oh, she hid it well, but I always knew there was a dark side to Ellida Vitae." He shook his head slowly. "Still, even so... I'm not sure I would have believed this of her." He laughed shakily. "I must admit, I had my eye on poor Trigo."

"Professor Fractalis?" Lily said disbelievingly. The Arithmancy professor was well-known for his timidity.

"I know, I know," he admitted. "But he's been acting so strangely lately-"

Lily giggled.

Malachite frowned. "What?"

She looked at him, and then across at Snape, and then she started to laugh. "Honestly, you people - you're supposed to be the smart ones here! Of course he's acting strangely. He's got a girlfriend!"

"Janeida?" Malachite seemed totally taken aback. "Surely not?"

"Yeah!" said Lily pointedly. "It's obvious."

"Not quite so obvious as certain people who insist on shouting about how certain other people are not their boyfriend," interjected Sev acidly.

"Oh, give it a rest, greaseball," Lily said, rolling her eyes. "Anybody would think you wanted him for yourself, the way you go on about it all the time."

"I think I can find it within myself to resist his manly charms," said Sev dryly.

Malachite was listening to the two of them bicker with a slightly lost look on his face. "You two... know each other?" he asked carefully.

"Oh yeah. We're old friends," said Lily breezily.

"Let's not exaggerate," said Snape.

"We solve mysteries together," Lily elaborated, sticking her tongue out at him.

"I solve. She follows me around and makes smart remarks."

"At least they're smart," she shrugged.

"Smarter than the average Gryffindor," he said, shrugging back. "That's not difficult."

Lily shook her head again. "Professor Vitae? Really? She was always so nice..."

"She seemed nice," Sev corrected. "But she also tried to poison you, if you recall."

"That was her?" said Lily, rubbing her head as if she was getting a headache.

"Well, she was the one who had hold of the Gryffindor badges before they were given to you. And she told you to keep it on after you'd noticed there was something wrong with it."

"No doubt she managed to slide the blame on that one over to me," observed Malachite dryly.

"Well, yeah. But that's because you're a Slytherin," Lily objected.

Malachite laughed shortly. "Not quite. Mostly, it's because I'm me." He sighed. "You see, Ellida was once a typical Gryffindor, much like yourself. We were both in the tops of our houses, and fighting it out for pole position within our year.

"Ellida always used to push herself too far to try and be the best. One year, there was a terrible accident, and she nearly died when a potion went horribly wrong. I was able to save her life because in my snake form, I was safe from the poisonous fumes."

"But if you saved her life, then-"

"Alas," Dumbledore took up the story, "poor Ellida would never accept that she'd made a mistake with the potion. She continued to insist that there had been sabotage, that Carnus had been coming in to kill her, not to save her. She blamed all of her teachers, too, for refusing to believe her version of events." He shook his head. "We worried for her, but when she returned to the school the following year, all seemed well again, and we thought she had got over the trauma of the incident. It appears we were wrong."

"Clearly, she never gave up looking for revenge," Malachite nodded. "The Death Eaters must have seemed the perfect way for her to achieve it. It's easy to join someone else's campaign of hatred when that's all you can feel yourself. Her new friends have no doubt promised her the chance to take her revenge on me and all her other enemies, in return for her aid." Oddly, he didn't sound all that threatened by the prospect.

"So what do we do now?" asked Lily, exhaling heavily.

"Nothing," said Sev. They all looked at him. "I engineered this theft for them. If we move against her now, I'll be the biggest suspect. We can't do anything yet."

Dumbledore looked grave. "Much as it pains me to continue to harbour an enemy within these walls, I fear the deception is necessary. Severus is our closest link to the inner circles of the Death Eaters, and we need to protect him if we are to have a chance."

"But what about Professor Vitae?" Lily wanted to know. "We can't just- we can't just let her get away with it!"

"We won't," said Sev quietly. "But it's all about the timing. It only takes one mis-step for us to lose it all."

There was a long silence, and the Dumbledore clapped his hands. "And now," he announced, "we must return to the festival. Secrets may keep, but impatient students won't, and soon we will be missed."

The four of them descended from the tower, and went their separate ways.

Professor Malachite cornered Sev one last time before the Hogwarts Express came to take them all home. "This is a dangerous game you're playing, Severus," he warned.

"I'm not playing," Sev reminded him.

"No, you're not," the teacher agreed. "And neither are they. Lily's right, you know. We're asking for trouble keeping a spy this close to us."

"But now we know she's a spy," Sev pointed out. "And that puts us ahead of the game."

"Maybe." Malachite frowned reflectively. "But I can feel things coming to a head. You and your Death Eater companions will be graduating next year, and somehow I doubt you'll be going out quietly..."

"I'll handle it," said Sev. "I always do."

"I hope you know what you're doing," said Malachite quietly. "I just hope you know what you're doing."

Then the last call for the train came again, and Severus headed for the platform to leave Hogwarts for the second to last time. He left Professor Malachite behind on the platform, looking troubled.