Nothing happened.
John saw a shadow of legitimate fear on Minerva McGonagall's face. He tightened his grip on his wand and scanned the ceiling yet again, looking for a shadow. Anything.
"There!" shouted Flitwick, waving his wand toward the rafters above the door.
The jet of red light struck a solid body where John had seen nothing. The creature turned a deep blue, revealing itself hulking with its back to them. A veritable rainbow of spells shot toward it, but it leapt onto the wall, evading them all and dropping the house elf to the floor. The ravaged body fell with a soft, sickening splat, and the creature scrambled to the left to avoid the yellow light from McGonagall.
"It is a Zhacan!" The Doctor exclaimed, running forward and squinting at it. He pointed the sonic screwdriver and pressed a button.
The creature let out a roar and scrambled down the wall to the open door. John pointed his wand at the doors, but spells flew from almost every corner. The doors slammed shut just as the creature's tail disappeared through them.
"C'mon," the Doctor shouted. "And don't aim to kill."
"It's tried to murder six students!" McGonagall shouted back as she magicked the doors back open. "The time for clemency is past."
"Zhacan are peaceful creatures. Something must be wrong with it"
The Doctor dashed out into the entrance hall, where a slightly stunned Jim Moriarty was picking himself up off the floor.
"Did it touch you? Did it sting you?" McGonagall asked, pulling him roughly to his feet.
Moriarty shook his head.
"Which way did it go?" the Doctor asked, spinning in a near-complete circle.
John kept his eyes on Moriarty, searching for the slightest hint of deception. The boy's eyes were wide, but there was a control in the terror depicted on his face that John did not like.
"I didn't see. It vanished. Right in front of me."
The Doctor nodded. "Right. So we split up. Minerva, Filius, pick a direction and a student to take along."
Sherlock held up a hand to stave off McGonagall's protests. "We know which way it went."
The Doctor studied Sherlock's face for several long seconds. "Okay. You lead."
"The children shouldn't come," Flitwick spluttered.
"We've been involved right along, sir," John said, his voice edged with the same steel he'd known last May when he and Neville had gone toe-to-toe with McGonagall to be allowed to stay and fight. "You'll need us."
"Filius, Minerva – you should stay here to keep the other children safe," the Doctor said, a note of finality in his voice. "I'll take these three. They know what they're about. We'll go after the Zhacan."
McGonagall was livid. "I will not allow you to put more students in danger."
"You're doing that right now," Sherlock said impatiently. "We know which way it went and probably where it was going. You're slowing us up. We'll have to catch up to it. If it gets away, we'll have lost our best shot at stopping it."
Flitwick and McGonagall both looked so torn that John almost pitied them. The Doctor nodded at Sherlock, who took off toward the front doors, Lindsay at his heels.
"You two stay here in case it doubles back," the Doctor told the professors. "Keep everyone inside. We'll be back."
John spared one look for Jim, who wore the same gleeful expression he'd seen on Sherlock's face when he was putting information together. But the Slytherin made no move to follow them as he and the Doctor pounded out the door after Sherlock and Lindsay.
The alien was nowhere to be seen, but Sherlock was already tracking across the yard, bent almost double to examine the grass.
"It came this way," he called back, motioning them forward without taking his eyes off the ground. "The weekend rains left the ground just soft enough for those claws to leave marks."
John didn't see anything other than grass that had been walked on by dozens of students over the weekend, but there was no point in saying so. They were following a fairly direct line toward the Forbidden Forest, angling away from Hagrid's cabin and toward the lake. Sherlock's self-assured pace faltered only once after they'd entered the forest and the markings became less distinct.
"It knew exactly where it was heading," he said as his half-run slowed to a walk. "The markings are almost gone, but we should keep to this line." He turned to look at the Doctor.
"You said you recognized it as a Zhacan – anything that can help us?"
"They hate high-pitched noises," the Doctor said, raising his sonic screwdriver.
There was a roar from the direction of the lake, just over the rise of land.
"Well, that got its attention," the Doctor said. "Alright, we're going to have to try and surround it. Zhacan like nests near water, so I bet it's got itself a nice little hideaway over there. If it feels outnumbered, it might go peaceably."
"Go peaceably where?" John asked, crossing his arms.
"If it is willing to leave this place, I can take it in the TARDIS to its home planet – or some other planet. Something made it attack people and I don't understand what it could be. They avoid humans. They're not aggressive creatures as a rule."
The Doctor retreated into a brown study. Sherlock waited about two breaths before breaking it.
"So should we go after it or let it hide itself even better than it already has?"
The Doctor shook himself. "I'll take one of you and go north, and the others can go around to the southeast. We'll try to keep its back to the water. If it's really a Zhacan, its invisibility should last only about 3 minutes at a time, and agitation causes it to lose the pigmentation. If it gets too close, just whistle as loud as you can or make some sort of high-pitched noise."
He looked the three of them over. "I'll take Lovejoy with me."
John couldn't help but wonder if the choice was because he actually wanted Lindsay with him, or because any of the other pairings seemed untenable. He gave Lindsay a salute and turned to head off with Sherlock.
They didn't speak as they made their way around the rise toward the lake, communicating with hand motions and jerks of the head. Sherlock's eyes were only half-focused, which could only mean he was thinking of something else. John tried not to worry too much about having Sherlock's attention divided – it stood to reason that half of Sherlock Holmes' attention would be at least as good as the average person's full attention. He had to chuckle at himself – had it really been a week ago that he'd been dreading spending more than five seconds in the Ravenclaw's company? The boy had surprised him.
"We're missing something," Sherlock muttered, following John's lead to crouch behind some brush where the rise leveled out.
John motioned him silent, peering around the branches to the lake. There was a stretch of shore that was well trampled and a depression in the side of the hill that couldn't quite be called a cave but seemed to be deep enough to hide the creature. A flash of movement caught his eye. The Doctor and Lindsay were diagonal from them, closer to the edge of the lake, taking up positions behind two large trees. Sherlock had closed his eyes and furrowed his brow, lips moving in silent monologue.
"Sherlock!" John hissed, elbowing him. Sherlock's eyes opened, and he glared at John. "This isn't the time!"
"Very well, then, I suppose it would be better to face down the killer alien without considering all the information."
John didn't bother responding. The Doctor waved once to get their attention, then motioned toward the cave, pointing toward himself. He drew a circle in the air, and motioned to the three students. John gave a firm nod. It had nothing to do with letting the adult take the risk and everything to do with putting the wand-wielders in a position to protect the one who know enough to dare to get close. Besides, he wasn't certain he could classify the Doctor as an adult.
"Get ready," he muttered.
Sherlock obliged insofar as he drew his wand and assumed a position that allowed for greater movement, and his eyes dropped a fraction of their distracted expression. John set himself up with a direct line of sight to the alcove and readied his wand. The Doctor stepped out from behind the tree and took several experimental steps into the open.
"Anybody home?" he called. "We're not going to attack, we just want to see you."
John didn't like the idea of that particular promise. Whatever the Doctor said about the Zhacan being peaceful, he wasn't above aiming to kill the creature.
There was no response. The Doctor took a few more steps. John let his eyes relax, focusing on detecting movement rather than details. The Doctor's slightly-swinging arms and the light breeze from the lake rustling a few leaves were the only things moving.
"Come on, then, it's rude not to acknowledge guests," the Doctor called, raising his sonic screwdriver. "I don't want to have to use this."
A sudden bank of blue to the left. John swiveled half-around the bush and trained his wand on the Zhacan that had materialized in the mouth of the cave. For a moment, the clear view of the creature froze him.
Winky had been right – the alien had bulbous red eyes that looked like those of a fly. The dark blue fur appeared short, which meant the mass of the limbs and torso had to be mostly muscle. It moved from a crouching position to stand upright, well above the height of a man, and swished its tail to curl around in front of its feet in a casually threatening way. John Watson did not consider himself a fearful person, but the sight of it had him thinking nostalgically of facing Death Eaters and acromantulas.
The Doctor seemed unperturbed. "Hello, there!" he said, smiling cheerfully. "Now, we're got to have a talk about your choice of home."
The Zhacan let forth a roar.
"Yes, I know, you came here in search of food. No one can blame you for settling where there was a ready supply. But you can't stay." The Doctor put his hands in his pockets and looked for the world as if he was reasoning with a petulant child. "You've done harm here. I won't allow it to continue."
The Zhacan made another noise, less ferocious and more questioning.
"You crossed the line. You attacked innocent humans. Children. Do you understand?" The Doctor said, his tone hard.
This time, even John could recognize that the sound the Zhacan made was meant to convey confusion.
"Don't play dumb. You know exactly what I'm talking about."
The Zhacan bared its teeth and lashed out. John didn't wait for the Doctor's reaction. He aimed and fired off a stinging jinx at the alien's chest.
The Doctor's horrified yell was secondary to John's concern that he be alive to make it. The creature stumbled back, and roared again, but immediately charged forward at the source of the spell. Sherlock flicked his wand and several roots came up to tangle around the alien's feet. It fell in a mass of limbs and claws, and scrambled to its feet again before John could draw breath.
"Calm down, all of you!" the Doctor shouted, fiddling rather desperately with his sonic screwdriver.
Movement to his right caught John's attention. Lindsay had dashed forward and shot a brilliantly blue spell at the creature. It landed under its second set of arms and set the whole creature into a rather grotesquely violent form of a dance. It careened away from John and Sherlock and back toward the water.
John took a moment to appreciate the fact that Lindsay had actually found a use for tarantallegra, which had always seemed a rather pointless jinx, but the creature seemed to have shaken the spell off almost at once. Lindsay backed away toward the cave, wand raised. The Zhacan let out its strongest roar yet and charged. The length of its stride was impressive. And terrifying.
John looked to his left to coordinate with Sherlock, only to find him gone. John swore. The Ravenclaw was halfway to the cave himself, attention caught by something John couldn't see. The Doctor was moving forward, eyes still on the implement in his hand.
"Don't cast any more spells. We can settle this peacefully!" he said, pressing a button the screwdriver.
Nothing happened.
The creature had drawn up at the sight. The split-second hesitation was the only diversion John was likely to get. He made his decision.
Lindsay was standing with wand raised, a look of indecision on her face. Sherlock had reached the mouth of the cave and was peering in. The Doctor was waving the sonic screwdriver around like a mobile phone trying to get a signal. In another two seconds, one of them would be cut to ribbons on those long claws.
John took three long strides and dove around the Zhacan, flinging himself down on his back in front of the Doctor and Lindsay.
"Protego!"
The shield charm that ballooned out from his wand knocked the Zhacan onto its haunches. John ignored the jarring his shoulder had received and held his wand steady, keeping the transparent barrier between them and the creature.
If it had a recognizable expression, John would have called it rage. The creature was positively mad, swinging all four of its limbs at the shield, which so far was holding up better than the rest of their spells.
"Good thought," the Doctor said, smiling. "I told you the screwdriver has been on the fritz. Apparently its only up for a couple of uses a day around here. Well," he said, smiling through the barrier at the Zhacan. "This is a nice way to settle things. Tell me, are you full Zhacan?"
John didn't need the Doctor to translate the sound the Zhacan made. It had no intention of answering any of their questions.
"You look the same. Exactly the same. I find it hard to believe that a mutation that powerful wouldn't have affected your form in some way. C'mon, you can tell me."
"Whatever else it is, I believe I know why it's been taking house elves rather than eating them in the castle," Sherlock said from behind them.
He sounded so matter-of-fact, John had to wonder if he realized they were facing down a killer animal and not discussing theory in potions. Then he heard Lindsay's surprised exclamation from behind, and saw the Doctor's whole body sag in surprise. He risked turning his head slightly to see what had caused such a reaction.
Sherlock was crouching at the mouth of the cave, shining his wandlight inside. Two pairs of bulbous red eyes stared at the humans in frank confusion.
"What the –" John hesitated to find the correct expletive.
"It's a mother," the Doctor said.