He didn't have much time to contemplate why the never-to-be-sufficiently-damned things might be swarming the castle before the Dementors noticed his and Minerva's presences on the tower and he had to turn all of his attention to pouring power into his Patronus. And even with him and Minerva standing back to back and casting with everything they had, he could still feel the Dementors' deadly chill starting to seep into his bones as the things pressed closer.

They couldn't risk fall back inside, not with the Dementors right on top of them. He didn't know for a fact that Dementors were enough like Lethifolds to latch onto a victim and use said victim to bypass wards, but he didn't know that they weren't either, and given the other characteristics in common between the two…. Instead, he did his best to ignore the feeling of creeping cold—as much as such a thing could be ignored—and focused on forcing them back up into the sky, back beyond the range at which they were dangerous.

Dementor after Dementor swept in, and he tried to concentrate on the ones in front of and above him, knowing Minerva was doing the same with the ones that came at her. But whether it was the adrenaline flowing through him or the harrowing effects of the Dementors presence, he was having trouble making his magic eye focus correctly, and every few seconds a Dementor would get closer than it should have on that side. At which point the chill would become almost unbearable, and he would have to bring his Patronus back in close to drive it away, which then let other Dementors as a group get closer to the two of them, and….

I will not die on Gryffindor bloody Tower! He shook his head, gritted his teeth, and made himself focus, sending his Patronus to kick the next Dementor that swooped down at him. Successful, he waved it over to charge at the next, spun it on a third, and then repeated the whole thing again. And still the things kept coming because Dementors couldn't be killed, only driven off temporarily, and for a few frantic seconds he thought that despite his efforts the cold would encompass him and they would lose the battle entirely after all.

But then the pressure seemed to lessen abruptly…not much, but when he forced another surge of power through his nearly-numb fingers, he actually drove the Dementors back far enough to allow him a quick glance around. At which point he found that he could see the glowing white forms of other Patronuses circling other towers now as well. Forcing the Dementors to split their attention and helping to keep the mass at bay. Thank Merlin.

He craned his neck to look over the wall, and other Patronuses seemed to be flowing out from various windows and upwards from the grounds…presumably the ones from the grounds were from professors who'd gone out when the alarms had sounded to bring in any students caught outside, and he felt some relief. Not much, though; there was so much ground to cover that he wasn't sure how much good it was doing. How many Dementors might already be down there—might already have been down there before the alarms had sounded. As little as he liked the idea of being a distraction, he hoped that his and Minerva's presences on this tower had been enough to keep most of the Dementors focused here rather than anyone who might have been down there, but there were just so many of the bloody things that there was no way to tell.

"Do we go back inside? Go down to the grounds?" he asked Minerva quickly when they finally managed to get a little more breathing room. Now that he was able to look around, he was able to assure himself that there no students—or, worse, the crumpled forms of what had once been students—here on this tower that needed to be taken to safety, but it was before curfew and a tolerably decent night out so who knew how many might have been caught elsewhere. Or how many professors were actually down there to help them; a few white forms hardly gave him a definite count. Whether their time was better spent holding the things at bay or going in search of said students….

"Not yet!" she said with a shake of her head, sending her Patronus to scatter a cloud of Dementors that seemed to be plotting a new charge with a sweep of its paw. "It'll take too long to get down there; we're more useful keeping as many of the Dementors up above the castle as we can!"

For as long as we can, he agreed with a quick nod, since her thoughts mirrored his initial idea. He waved his wand, and his Patronus spun on a Dementor that ventured too close and then charged off to drive upwards another that had started to stoop towards the grounds. Now that the first rush had thinned, their attention divided between the various towers, his and Minerva's Patronuses should be able to hold the space around Gryffindor Tower for some time.

And the Patronus spells didn't really have to do much beyond buy them time, he realized after a minute of thought. Well, that and keep them alive, obviously, but they only really needed enough time to get all the students safely inside and away from the windows. After that was done, everyone in the castle could take shelter and wait until help arrived. Presumably, Albus would have called for the Order, or at the very least someone at the Ministry—as useless as they generally were—as soon as the alarms had sounded. And once everyone was inside and the doors and windows were shut and locked, there was no way that those things could hope to breach the castle wards.

A Dementor dove downwards with an inhuman scream, and he flicked his wand to send his Patronus circling around to chase it back off. Well, he hoped that they couldn't breach the wards, anyway. How many Dementors were there at Azkaban? A hundred? Two hundred? More? More was just barely possible, given how many of them now seemed to be gathered around the school.

A horse charged upwards suddenly, disrupting a cloud of Dementors that had gathered just beyond the positions he and Minerva's Patronuses had taken up, an otter hard on its heels—hooves?—helping to scatter them still further. Almost immediately after that, a hawk and a dog shot out of the other side of the tower and drove another wave of the things back.

With another surge of power and a frown at the new arrivals, Severus twisted to scan the rest of the castle a little more closely. That was definitely the white form of Aurora's bat circling the Astronomy tower—it was one of the Patronuses that he'd noticed just a minute before—but was that a…swan?...beside it? Miss Cho's Patronus is a swan, he remembered abruptly, which meant that the four Patronuses that had just appeared around the tower probably belonged to Gryffindor members of Harry's little club.

He frowned up at the four unknown Patronuses. The Weasleys would account for the lot of them neatly, although Longbottom or Granger or any of the other obnoxious little lions that Harry called friends could just as easily be casting the spell as well. If nothing else, if the Weasley twins didn't have twin Patronuses I would be greatly shocked…although I suppose that would be one sure way to tell them apart.

Precisely whom the Patronuses belonged to was irrelevant now, though, and he sent another surge of power to his own Patronus and before twisting to look around the castle again. Severus didn't have a clear view of the tower nearest the headmaster's study from here, but the large birdlike form circling around that area was probably Albus' phoenix, and he had no doubt that, short of force being used to prevent it, Harry's stag was somewhere in that general direction as well. Even if force was used to prevent it, the little idiot would probably still find a way to get himself involved.

Another Dementor swept inwards, and with a flick of his wand, Severus made his Patronus kick it. The kick didn't actually impact anything of course, but it made him feel marginally better as the Dementor shrieked and swept back up into the sky to join its fellows.

He shook his head. What were these things doing? The presence of the other Patronuses was finally giving Severus time to think, at least, but he still couldn't come up with any reasonable ideas about what the things might be planning. Openly breaching the wards…no, it couldn't happen. Maybe—probably—it could have a few years ago, but Albus had spent several weeks of the summer after the Dementors' last little visit upgrading those wards, and they had to know that they couldn't just break through. Perhaps if the things approached with stealth they could have taken a few students—and as much as he didn't want to consider it, it was entirely possible that they had before the alarms had sounded—but now that they had been seen, there wasn't a great deal that they could accomplish. Unless they could attach themselves to victims, in which case they might be able to sneak inside that way. It was not a pleasant thought

Something flickered in the corner of his good eye, and he twisted just in time to see the Patronuses around the Astronomy tower disappear abruptly. There was a sickening shriek of pleasure from that general direction almost immediately after, presumably because of said disappearances, and Severus sucked in his breath sharply as he realized that many of the other Patronuses that he'd seen not minutes ago were now missing as well. If they are getting inside somehow…Merlin help us all.

He had thought that he and Minerva had faced the worst of it during the initial attack on Gryffindor Tower, but they were two of the strongest in the castle when it came to defensive spells. The Dementors could have changed their tactics, swarming and taking the other towers one by one, in which case—

"Expectro Patronum!"

The shout startled him as much as the wolf that pounded upwards from behind him, but even if he hadn't recognized the voice, the thump and clatter that followed immediately after would have identified its owner. "Miss Tonks, so nice of you to join us." And attempt to give us heart failure. Even if her presence wasn't entirely unwelcome, surely she could have found a better way to announce herself.

"Wotcher, Professors." Severus glanced backwards to find her shoving herself back to her feet and brushing off her robes, managing to look entirely too cheerful given the circumstances. "We've got Aurors sweeping the castle and escorting all of the students and professors to the Great Hall. Shacklebolt says we're to give them five minutes more to make sure that this tower is clear and then join them."

That explained the missing Patronuses, at least, and he breathed a slight sigh of relief and then almost absently sent his Patronus after a Dementor that had ventured a little too closely. At least there was some sort of plan in place.

The dog and the hawk vanished abruptly, the otter and the horse following a few seconds later, and Severus nodded slightly as he considered Tonks' words. All things considered, it was even a reasonably good plan. The Great Hall was large enough to hold everyone, and although it did have doors to the courtyard and windows along one wall, all of them shut and latched firmly and were anchored to the wards, easily defensible without risking tiring the staff. While the dungeons might be slightly more defensible given their lack of windows, there was also nowhere large enough for everyone to gather together—well, nowhere aside from the Chamber of Secrets, anyway—and he could see no logic in splitting up any more than necessary. Whatever the Dementors' plans were, assuming that the creatures made actual plans beyond sense-the-soul suck-out-the-soul, which, Severus supposed, he couldn't guarantee, they couldn't possibly hold the castle under siege for that long. Especially if Aurors were already here.

When the given five minutes had passed, the three of them made their way down out of the now-deserted tower quickly and found Kingsley waiting for them at the bottom of the staircase.

"Are all of the students accounted for?" Minerva demanded as they hurried towards him. "And the staff?"

"All of the students, yes, and you two are the last of the professors to arrive," he said with a nod. "There were six students and a professor that the Dementors managed to pin down before anyone could get to them to help, but as last I heard there was no permanent damage and your mediwitch says they'll all recover fully in time. Everyone is being given hot chocolate as a precaution, but for the time being it looks like the situation is under control."

Severus snorted and didn't bother to hide it. In his experience, more often than not, that was when everything went exceedingly wrong. As Kingsley should well know. He didn't have time to voice his opinion though, as the four of them entered the Great Hall and were promptly run through by a hedgehog Patronus, its spines bristling far more threateningly than any living creature's every had.

"Yes, Alastor, I'm hiding a Dementor in my pocket." Although that would neatly take care of any Dementors that had thought to ride one of their victims through the wards. But he wasn't going to reinforce Alastor's paranoia by acknowledging that.

Alastor muttered something that sounded suspiciously like 'Sounds about right' before barking, "Constant vigilance!" in their general direction and stomping off towards the shuttered windows.

"Professor?" a voice asked from beside him before he could come up with an appropriately snide response.

"Mr. Potter?" Minerva asked, her words overlapping Severus' "Harry?"

"Are you all right? Were you injured in the attack?" Severus asked when Minerva didn't immediately continue. He had assumed that the brat had been with the headmaster when the attack had occurred, but now that he thought about it, it would be just like him to have been out on the grounds on his way to visit Hagrid or something equally ridiculous given the time that the attack had occurred. Hagrid….

Severus cast what he hoped was a discrete glance around and felt his stomach twist slightly when he didn't see any sign of the half-giant. But Kingsley had said that all of the staff were accounted for so he must be around here somewhere. Perhaps he'd been exposed to the Dementors…one Professor had been, and it being Hagrid made a disturbing sort of sense given the location of his hut. Severus would have to remember to seek him out later and make sure that he really was okay.

"Hm?" Harry shook his head, clearly not even noticing Severus' momentary distraction. "No, no, I'm okay. I'm fine."

Minerva glanced at Severus and then followed the two Aurors as they moved past Severus and Harry to join Alastor by the shuttered windows, and Severus signaled Harry to step to the side. "That was strikingly unconvincing. Would you care to try again?"

"Really, Professor. Madame Pomfrey checked me and everything. I just…I don't like Dementors." He shrugged awkwardly. "I mean, I know they can't get inside, but…I still don't like them."

Severus wasn't entirely convinced, but he knew Poppy well enough to know that she wouldn't be letting the brat wander around if he was injured. "Reasonable enough, I suppose. Even for a Gryffindor. But you are correct, they can't get inside, so was there something that you wanted?"

"There are a bunch of us—students—that can cast the Patronus. Remus taught me, and I taught them all in DA. We can help." He shook his head. "I tried to tell Professor Dumbledore that, but he just told me to hurry down here."

Albus behaving reasonably where the Golden Boy is concerned; I must alert the news media. He nodded slightly. "That is good to know. If we need your assistance, we'll inform you."

Harry's jaw set in a decidedly stubborn look. "Professor—"

"We will let you know," Severus said with a glare. "And in the meantime, you will not be rushing off on your own in any kind of senseless stunt. We are not in any danger at the moment, and there are more than enough professionals on hand to deal with the situation without throwing a dozen students into the mix." Or more. He still didn't know how many of the little brats were members of this 'DA'. "Now, why don't you go and get some hot chocolate from one of the house elves and then find a seat with your friends. Helping to keep everyone calm is the best thing you can do right now."

"I'm not thirsty," Harry said with a half-scowl.

"I don't recall asking if you were. Go."

"Poppy, did you drug the students?" Severus had to ask as he looked around at the mass of sleeping forms. Through judicious use of transfiguration, the Great Hall had been transformed into something of a massive dormitory, and far more of the students had fallen asleep than he would ever have expected. The fact that Dementors had the castle surrounded wasn't exactly a secret, and he'd have expected nerves alone to keep most of them awake.

She sniffed. "I did no such thing. I merely added extra calming draught to their hot chocolate. Perhaps you should have some."

"Well done," Albus said, before Severus could reply to that suggestion. "There's no sense in depriving them of a night's rest when the situation is under control."

Severus very much wished that people would stop saying things like that, but Alastor's bark of 'constant vigilance!' rang out before be could say anything. It is a sad day in this world when I actually agree with anything that comes out of that lunatic's mouth.

"Keep your voice down," Poppy ordered sharply, twisting to glare at him. "Honestly, I didn't give them anything that can't be canceled with a simple spell if we need to get them up and moving quickly."

"Yes, this may be necessary. I foresee great trials in our future," Sybil said, coming up behind Severus. "In fact, in the crystal the…."

And she couldn't have been the one exposed to the Dementors, because? Poppy hadn't let him get more than a glance at Hagrid—or the six students who'd suffered severe exposure—insisting that it was better to let them rest peacefully, thoroughly dosed with whatever she'd given them, but that glance had been more than enough. For someone who was usually so lively…. He shook his head and forced himself to return his attention to the current conversation. Poppy had said that they would all recover.

"Well, it seems that those trials may already have begun," Albus said, cutting Sybil off before she could elaborate further. "Could you take one of the Aurors and make sure that the floo in my office remains secure? If it turns out that I'm wrong about the level of danger we are in and we do need to evacuate, that will be the surest way."

Not the quickest, Severus knew, not with the number of students that would have to go through, but it did remove any necessity of going outside. Of course, that assumed that they would need to evacuate—and that the floo is secure—which…. He shook his head. None of this makes any sense. He made another circuit of the room, checking on his Slytherins out of habit. Fortunately, they were all accounted for; it was four first-year students from Ravenclaw who'd been paying a late visit to the greenhouse and a seventh-year each from Gryffindor and Hufflepuff who'd been snogging down by the lake who'd been caught out by the Dementors. He happened to glance over at the mass of Gryffindors as well—most of the fifth year lot seemed to be huddled together, not that he particularly cared—before going to stand with Minerva, Kingsley, and Alastor.

"—t's a distraction, I tell you," Alastor was muttering as he stepped up beside Minerva.

"Yes, Alastor, you said that before." Kingsley shook his head, and if his expression was anything to go by, Alastor had probably said that several times before. "But a distraction from what? For what? The castle wards would have locked down when the alarms sounded, and a dozen other Aurors and a cadre of Hit Wizards will be here first thing tomorrow morning, at which point we'll have enough Patronuses available to drive the things all the way to the Continent if we want to."

Alastor shook his head. "Maybe they've already done it. Whatever it is they came to do. Done it and let the panic cover their tracks."

Conspicuously leaving out whatever 'it' might be, Severus noted…if 'it' had been capturing Harry or smuggling a Deatheater army into Hogwarts or something like that, he might actually have considered it a reasonable suggestion, but since everyone was accounted for and they would have been alerted if anyone unexpected was in the castle…well, it just didn't make sense.

"What if what they came to do is cause a panic?" Minerva suggested, peering through the window shutters and then shaking her head as she turned back to the group. "No one is putting much faith in the Ministry at this point, and if people hear that Dementors attacked their children at school..." She shook her head. "Well, it wouldn't leave them much faith in us—or, more specifically, Albus—either. Especially if the attack was at all successful. We were lucky beyond belief that it wasn't and no permanent harm seems to have been done."

Severus agreed wholeheartedly with that, and while he didn't see the Dementors coming up with such a scheme on their own, the idea of their presence being used to incite even a minor panic in the Wizarding world did make an insidious sort of sense. Any influence that Fudge and his cronies had drawn away from Albus over the past summer had been well and truly regained after the rather spectacular public return of the Dark Lord and the incident with Umbridge, and if someone wanted to counter that, showing that the school was vulnerable was a fairly obvious place to start. Not that he thought that even Fudge would be stupid enough to enlist Dementors, but one of the Dark Lord's cadre with a position at the Ministry or designs on such a position in the future—and given some of the newest residents of Azkaban, regaining influence at the Ministry would be a priority—…. Bloody hell, that's just what we need.

He shook his head and spoke in support of Minerva. "Even if they didn't manage to cause any permanent harm, the damage may already have been done. The Dementors did attack the school. And even if there's no way that any reasonable witch or wizard could expect that Albus would have known about and averted the attack in advance, I think we are all well aware that people are quite often unreasonable when their children are involved." For example, Merlin help us when Molly Weasley hears about this.

"I agree. And given that I called for Aurors immediately, the word is probably already spreading," Albus said quietly, joining the group with Tonks beside him. "I expect that the howlers will start arriving with the dawn."

"I still say that can't be all that they hoped to accomplish," Alastor said with a scowl.

Albus shook his head. "You were never one for the political games, old friend. I can't say that I ever expected this, but I was expecting some sort of challenge. Although I think we can be reasonably sure that Voldemort was behind this as opposed to anyone else."

That was no real consolation as far as Severus was concerned, but the more he thought about it, the more sense it made. Alastor still didn't entirely look convinced, but then, he probably wouldn't be happy unless the evening climaxed with an army of Deatheaters pouring into the castle through the drains or something. Which…. "I presume you did lock down all entrances of the castle when the alarms sounded, correct?" There was no harm in making certain.

"As soon as everyone was inside," Albus said with a nod, "and I've had the house elves and several of the portraits that I trust do a sweep as soon as the castle was secure to ensure that no one…unexpected…managed to get inside during the rush."

"Would have been safer to shut the doors immediately," Alastor said.

And condemning those caught outside; what an excellent plan.

"Yes, I'm well aware of that," Albus said before Severus could voice his opinion, "but I wasn't willing to risk trapping any of the students out on the grounds given the danger that even a single Dementor poses." Albus looked around. "We're secure enough here that I think as long as we take watch in shifts, we should be fine for the night. And tomorrow, after the Dementors have been driven off, we'll see how much damage has been done."

It wasn't precisely a good solution in Severus' opinion, but he didn't see another option at the moment, so he nodded and dispersed with the rest of the group.

"Professor?"

Severus jerked, startled. The Aurors had taken up positions at the doors and windows and declined—well, except where Alastor was concerned, anyway, probably because he hadn't given them a choice—Albus' offer for the faculty to help spell them, which had left him at something of loose ends. A few of the other professors had actually manage to go to sleep, albeit with the aid of Poppy's calming draught-laced hot chocolate, but Severus had absolutely no desire to lay down. But the book he'd summoned from his quarters hadn't been holding his attention very well, either, unfortunately.

That didn't mean that he enjoyed being startled out of it, though, and he scowled at the brat. "Mr. Potter, may I ask what you're doing up at this hour of the night."

"Can't sleep."

"May I suggest more hot chocolate?"

Harry shrugged, taking a seat, uninvited, on the other end of the cot that Severus had claimed. "I told you before, I'm not thirsty. I gave mine to Ron."

Well, that explains why he was still fully conscious, anyway. Severus remained silent.

"Why did they come here?" Harry asked after a moment. "I mean, it's not like third year, when they were searching for someone. They're just...here."

"The question of 'why' is one that we would all like to know the answer to, I'm afraid."

"Is it because of me?"

"I believe I've pointed this out before, but contrary to popular belief, the world does not actually revolve around you. The current theory is that this may be the first step in a political game."

"Hm." Harry made a face at that.

"If you'd like another theory, please feel free to speak to Professor Moody. I'm sure he plans to go searching for the invading horde as soon as he's finished with his watch." As soon as the words were out of his mouth, Severus realized how inappropriate they were—he and Alastor might spar privately, and if Alastor had been in front of him he'd certainly have suggested something similar, but saying something like that to a student was completely unprofessional—and shook his head. "However, if you'll excuse me, I have reading to do." And perhaps he should get some un-potioned hot chocolate, just to make sure that the Dementors hadn't affected him more than he'd thought.

Harry didn't look very convinced. "Can I at least sit here? Ron and Neville and everyone are asleep. I won't bother you."

Unlikely. "I suppose. If you will drink some of Poppy's hot chocolate. You've dealt with Dementors before; you should understand why it's being offered."

"I guess." He glanced to the side. "Dobby?"

The psychotic house elf that Severus had met before popped in almost immediately, and he was far happier than any creature should be to hand over a glass of chocolate to someone. But at least Harry accepted it, and before the creature could disappear again, Severus asked for one of his own. Although he did carefully specify plain hot chocolate. While he waited for it to arrive, Severus snapped his fingers, summoning a second book and passing it to Harry.

"What's this?"

"As I'm sure you don't recall, your assignment for tomorrow—" or whenever the next Potions class was going to be held—"was a summary of the various uses of Bezeor root and its closest relatives. If you aren't going to sleep, you might as well use this time to actually read the chapter as opposed to sitting there worrying."

"I read it," Harry objected.

More likely Miss Granger summarized it for you, Severus thought, but he settled for a slight sneer. "Will wonders never cease? In that case, you may feel free to use your time to read ahead."

Harry muttered something that probably wasn't complimentary, but after a minute he took a sip of his chocolate, pulled his legs up onto the cot, and opened the book, and after a moment Severus reopened his book as well. I know I just read this section, but….