(Minor corrections to text, March 2013)

Disclaimer:

I am not J K Rowling. I do not own Harry Potter.

I am not H P Lovecraft. I do not own shoggoths, The Necronomicon, the Dunwich Horror, the power of Ibn-Ghazi, or various other accoutrements, families, or trappings of the Cthulhu mythos.

Warning! Shoggoth involvement! Messy character death implied. This story is identified as 'horror'.

Further note: This is set in an alternate Harry Potter universe, which crosses over with that of the Cthulhu Mythos. Some details such as which exam has just been sat and how many others were present vary a bit. Others such as what the heck is going on that year vary quite substantially.

There were ripples spreading on the surface of the lake, as the group of fifth year students, lost in their own little dramas, stood and shouted at one another, and traded spells. Ripples which were gradually increasing at their point of origin, as the tensions rose on the shore. There was something unnatural in the air right now about that spot on the shore, otherwise on such a sunny day, just after an exam session was over, there might have been more students than this handful of Gryffindors and a lone Slytherin assembled.

The fifth year History of Magic exam was just done, and the gang of four Gryffindor students known as 'The Marauders' had stalked and attacked their favourite target from Slytherin, Severus Snape.

Lily Evans swept onto the scene, intent upon making a speech which would lead to the breakdown of a beautiful friendship and over two decades of misery for the majority of those involved, when she was mercifully cut short as the source of the ripples started to break the surface of the lake a hundred yards or so out. It was something which was making doggedly for the shore.

She closed her mouth and turned to gape.

"What in Merlin's name is that? That's not the giant squid."

Out on the lake a small pile of dark bubbles of something like oily black scum had surfaced, and as it headed in their general direction, the pile seemed to swell and expand as if more of it were rising up from underneath the water.

The others turned and gaped. Even Severus Snape, dangling in mid-air, did so.

"Good question, Evans." James Potter stared.

"Maybe it's one of Snivellus' bogies." Sirius Black sniggered.

It was closer now, and a strange bubbling whistle was starting to emanate from it. Whatever it was, it was big.

And suddenly Severus Snape blanched.

"It's a shoggoth. I've been telling you idiots for weeks that Professor Whateley was more than just a kind professor who was here on a one-year contract from the States. Put me down, you idiots! Put me down!"

There was rising panic in his voice.

"It's just a big pile of scum. Or maybe one of Professor Slughorn's potions experiments gone wrong." James Potter turned to Severus. "Nothing to be scared of at all."

"…Elly-lee, elly-lee…" The whistling bubbling sound was getting stronger as more and more of the creature emerged from the water.

Lily frowned and sent a liberacorpus in Severus' direction. He dropped to the ground.

"How big is that thing, anyway?" Sirius pondered. There were now a good half-dozen feet of it visible, and it was surging faster and faster as it came closer to the shore.

"You… imbeciles." Severus was almost shrieking, as he desperately scrabbled and retrieved his wand. "Haven't you ever read The Necronomicon? Or those old reports Lovegood sent to the head of each house six months ago? It's too late to outrun it now. Or get a message to the professors. It's approximately fifteen feet of nightmarish, head-ripping, death."

"Oh, relax Severus, it's just a big pile of froth and foam. Or maybe you're scared because you think it's a giant shampoo monster." James Potter jeered. He flicked his wand at the approaching menace. "Incendio. Look: there, no problem."

The jet of flame which engulfed the creature for a moment left it steaming slightly, but otherwise seemed to leave it little harmed.

"Ah." said James Potter. "That didn't work as well as I thought it would. Incendio! Incendio!"

The next moment, a barrage of fire spells were flying from the wands of Potter, Black, and Pettigrew at the creature, as the sound it was making increased. It slowed slightly under the barrage of fire spells, but otherwise did not seem much deterred.

Remus Lupin tried a couple of stunning spells on it, but to little apparent effect. Lily sent some slowing jinxes which may or may not have affected it. It was difficult to tell.

Severus was desperately pacing up and down, muttering to himself.

"What are you doing, Sev?" She shouted over the barrage of spells and the 'kelly-lee, kelly-lee!' noises the creature was making.

"I'm trying to save us all by remembering what the devil I know about the wretched things. Unfortunately I do not have an eidetic memory, The Necronomicon is vague at best, and it's more than two months since I last reviewed the material Lovegood supplied. And to be frank, I was more worried, with a name like Whateley, with how to survive if our illustrious defence professor took it into his head to try and recreate the Dunwich Incident. I have half a dozen sachets of powder of Ibn-Ghazi prepared back in my quarters, and some notes on an appropriate incantation which might be sufficient to vaporise a reprise of the Dunwich Horror if all else fails."

"Tekeli-li! Tekeli-li!" The creature surged ashore. It was huge and nightmarish, a seething, boiling, roiling bubbling mass of darkness. It towered a good fifteen feet tall, and now it was on terra firma it started to shape itself, extending crude pseudpods, or other 'limbs' as it bore down.

"We need something which will work here and now against a shoggoth, Sev." Lily shouted back, between spells. By sheer weight, it bulldozered its way through a hasty transfigured iron fence Sirius threw up across its path. "Think!"

"Ice!" Severus finally snapped his fingers. "The only reported known land colony lived in thermal caverns under what for the sake of simplicity we'll pretend is the South Pole. But they never spread, because they don't like extreme cold."

"Freeze it!" Lily called, instantly switching her own tactics as Severus finally started casting, throwing ice-spells himself.

It was too late for Peter Pettigrew though. The shoggoth was close enough now to extend a couple of pseudopods in Peter's direction with horrible speed, and to seize the young wizard.

He gave a strangled cry which was abruptly cut off, followed by a horrible snap and rending noise, followed by a couple of wet thumps.

Sirius stopped casting and retched, violently. Remus hesitated a moment and blanched. The others paled, but kept the frantic barrage of spells going.

"Tekeli-li! TEKELI-LI!" the shoggoth shrilled its triumph, but it was a triumph that was to be short lived. Under the torrent of ice spells now hitting it, it was starting to slow noticeably, and it had come far enough ashore that it could not retreat to the lake.

At last it stopped there, frozen, a gleaming black malevolent mass encased in ice, barely quivering.

And then rather belatedly two professors arrived, Slughorn and McGonagall, wands out. There was something not quite right about their eyes though.

"Well done, students." Professor McGonagall said. "Fifty points to Slytherin and two-hundred and fifty to Gryffindor." Then she appeared to notice Peter wasn't actually moving, and his head was several feet remote from his torso. "But a hundred points off Gryffindor for Pettigrew not surviving."

Slughorn pointed his wand at the frozen shoggoth and muttered a variant blasting curse, and suddenly frozen chunks of it were flying everywhere.

"That takes care of that. Good charm work, Evans." he muttered.

"Professor…" James began to address Professor McGonagall, the Gryffindor head of house.

"What Potter was about to say is when you send Filch out to clear up Pettigrew, could you instruct him to be careful, since it was more bad-luck on Pettigrew's fault than lack of skill or cunning that cost him his life?" Severus said. "It wouldn't be fitting to see him fed to Hagrid's pets."

"Oh." said the head of Gryffindor. "Yes, well, Mr. Snape, I shall consider your request very carefully."

James had started to splutter an objection to Severus' speech, but shut his mouth when Lily laid a warning hand on his arm. Now he frowned.

"We'll go down to the dungeons, to have a celebratory butterbeer, if that would be alright?" Severus glanced at Professor Slughorn.

"Oh what, yes, fine m'boy. Ten points for inter-house co-operation."

Safely ensconced down in the dungeons, twenty minutes later, with every warding and privacy spell that Lily and Severus between them could throw over the disused classroom they were occupying, the surviving three Marauders, Lily, and Severus were taking stock.

"Whatever's going on, someone's imperiused our head of house!" James Potter was outraged. "There was no way she was acting even remotely normal."

"If you're lucky she's been imperiused, Potter. There are worse things in The Necronomicon than the Imperius Curse. Of course you have to be an insane genius to be able to understand them sufficiently to apply them, which I am not, but…"

"Sev. It doesn't matter what form of mind-control has been used. The point is something's been done to our head of house and to yours too."

"Well the actual specifics might…" Severus started to quibble with Lily, but trailed off and shrugged. "Okay, I see your point: in the general context, it probably doesn't matter much."

"They were acting as if it had been some sort of training exercise." Sirius said. "And I caught McGonagall's gaze; she seemed disappointed that if anyone had died, it was Peter who had done so. There's some sort of plan going here."

"It's something supposed to happen today, if it's not happening already." Severus said. "They'd have put the school under lockdown otherwise, to stop news getting out."

"We don't know they haven't." Remus commented. "We haven't tried to floo-call or owl anyone."

"The defence professor, Whateley, is almost certainly the main motivating agent here at Hogwarts." Severus said. "Given he's been living in the Astronomy Tower with Sinistra since the start of the school year, she has to be in it up to her neck, too, either willingly or otherwise. We know they have the heads of two houses at least temporarily under control. They also have either the headmaster's blessing for what they're doing, they have him under control, or he has no idea what's going on."

"Professor Dumbledore wouldn't willingly…" Lily began to object.

"He might." Sirius said, looking slightly guilty. "You don't know half of what he's prepared to do, Lily, if it fits in with whatever his current plan-of-the-moment is."

"And assuming our Professor Whateley is related to one mentioned in several copies of newspaper articles Lovegood sent, it seems likely Whateley has read The Necronomicon and may have a mind sufficiently unhinged that he had and can employ some of the things in it." Severus went on. "The good news is that what's in The Necronomicon could be considered 'ritual magic' which takes a long time to prepare for and to cast. Some of it is highly circumstantial. The bad news is it goes way beyond regular wand magic. We're talking tearing the fabric of time and space here, and unleashing cosmic horrors which make the thing we just fought out by the lake look like chickenfeed." Severus sighed. Then he scowled at James. "It would have been helpful if you'd accepted the truce I offered in November, as soon as I started to suspect Professor Whateley. There was no way that what he did at Hallowe'en was as harmless as he pretended. We could be in a much better informed and prepared position now, if I hadn't had the Marauders on my back all the time."

"Yeah, well, I didn't know that our defence Professor was going to be involved with this." James protested. "We still don't know that it's him for certain. I mean, you and your Slytherin friends could have cooked up that thing…"

"And put their own head of house under possible Imperius?" Lily glared at James. "Honestly, I can't even begin to imagine how you get the marks you do in exams if that's an example of your brains."

"Whilst I'm a fan of Potter put-downs, this isn't the time or place, Lily. We need to determine what Whateley and Sinistra are doing, and then put a stop to it." Severus said.

Author Notes:

There are no 'witnesses' to the scene by the lake in this universe at the start of the incident - just the Marauders, Severus, and Lily. The sense of 'something unnatural' in the air is keeping anyone else away.

'Lovegood' is in this case Xenophilius Lovegood. I have no idea when he attended Hogwarts, but for the purpose of this story, I've assumed he's already left, and is pursuing interests in cryptozoology.

Slughorn and McGonagall were acting odd because they were under someone/something else's control.

Apologies if the shoggoth is behaving differently (or is vulnerable in ways one might not normally be) from whatever may be 'normal' but I'm going by At the Mountains of Madness here, and it seems to me, based on that, that shoggoths don't like the cold...