Disclaimer: Everything that can be found in the Harry Potter novels, belongs to JK Rowling. The rest is mine.

Thanks very much to Elissa for beta-reading this chapter.

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Alligatorem Cognoscere
Chapter One: The Alligator's Legs

He's in the water, you're in the water. Who needs to be afraid? Your eyes widen when you see how fast the alligator swims to get a rat. You should be afraid.. Shouldn't you? You start to swim, and hear the alligator coming after you. "I'll never make it," you think. "Even if I do reach the land, he'll run, and eat me.." Will he? You reach the land, and without looking back, you run. Run! A branch makes you stumble, and you fall on the ground. "This is it," you think, "This is the end." You turn on your back, and to your surprise the alligator still is far away from you. Astonished you look at it, and see he's going terribly slow. Sharp teeth, yes, but very short legs.

*~*~*~*

"A great welcome to you all!" Albus Dumbledore spoke to the hundreds of students. All of their faces were turned to the man, who in spite of his age talked and acted like somebody half his age. Somebody like Cassius Ingaster. He listened to the speech of the Headmaster, hoping he would soon announce the meal. Cassius hadn't eaten since yesterday night, in the aeroplane. It had been a motley collection of some junk food, vegetables, and pieces of fruit. He wondered what it was that drove the muggles to eat things like that. His thoughts were suddenly interrupted.

"I'm glad to say I've found another DADA teacher, who was willing to teach at Hogwarts. Cassius Ingaster!"

Albus gave Cassius a friendly smile, and he stood up to receive the applause. When he sat down, Albus spoke some final words, and then gave the signal to let the meal appear. Cassius couldn't control himself any longer, and he grabbed everything that looked tasty, to hastily put in his mouth. When he saw some colleagues staring at him, he swallowed some magical beans, and continued his meal, but now somewhat slower.

Still he felt a piercing gaze, and when he looked up, he saw a man with a hooked nose, pale skin and black, greasy hair, staring at him. As if the guy didn't look bad enough, his expression was one of utter loathing. Cassius gave the man a smile, but he only turned his head away.

"Man, what's his problem?" Cassius muttered, while pouring some juice. The woman next to him must have had perfect ears, because she could hear Cassius through the laughing of Dumbledore who had just heard a good joke from Professor Flitwick.

"Something wrong?" the woman asked.

"Who's the man with the-" Cassius hesitated. Maybe 'man with the greasy hair' wasn't a good description. "-black robes?"

"That's Severus Snape, he is the Head of Slytherin House, and Potions Master here." She offered her hand. "Minerva McGonagall, deputy mistress."

"Cassius Ingaster." He didn't think it was necessary to tell her his job. "Why is he looking so acid?"

"Because he is acid."

Cassius saw that it hadn't been her intention to say that, because she quickly shut her mouth. He raised his eyebrow, but didn't ask any further.

His hunger had been allayed, and when he put down his napkin, the tables emptied, and he realised they had been waiting for him to finish the meal. The students left to their dormitories, and Cassius wanted to leave to his own room too, when he remembered there was a meeting in 10 minutes. He sighed, and headed for the teacher's room.

Most of the teachers were already in there, and Cassius sat down on a chair around the large marble table, which was standing at the centre of the room. When Albus Dumbledore came in, the rest of the teachers sat down too. Dumbledore walked to the head of the table, and took his place there, after checking everybody was present.

"I shall not beat about the bush." His face was friendly, but his voice stern. "Voldemort has returned, and even though every student is aware of that, I want them to feel safe. So I want to discuss when and where patrols will take place, and who will be conducting them. Minerva, will you take the Hufflepuff section? That's next to your room."

Cassius tried to listen to how everybody was being divided over the different sections of the castle, but he hadn't slept for at least 24 hours, due to the patrolling he had had to do in the muggle aeroplane. Because he was fully concentrated on keeping his eyes open, he didn't notice Albus calling his name. He startled when somebody shook his arm. Confused he looked up.

"I'm sorry, what were you saying?" he muttered.

Some teachers stifled some chuckles, and Cassius had definitely broken the tensions. The serious look in Dumbledore's eyes had disappeared.

"I said that you will take the east tower, as from tomorrow, of course. It is clear that you've done an excellent job staying awake on the plane, in order to protect the muggles. Please take a rest now, or you won't even be awake during tomorrow's lessons."

Cassius nodded, gratefully, and with a scraping noise he shoved away his chair to give him room to stand, and left the table, receiving compassionate looks.

It was getting dark already, and Cassius had to light his way with his wand to find his way to his room. It was located at the end of a large corridor, in the north tower. When he arrived there, Cassius stared at the door. With a sense of disbelief, he touched the letters of his name, which were engraved in the door. He unlocked door with a spell, and walked into the huge room.

Cassius had been in the room once before, to unpack his stuff, but it still fascinated him. On the left side stood a large bookcase, with loads of books. On the right side were his bed and his desk, and opposite the door was a fireplace, with a large couch and a table in front of it. Next to it, there was a door that led to a small bathroom.

Cassius walked straight to his bed, and let himself fall on it. He had everything that was needed to fall asleep; A warm bed, an empty head, and he was tired. Still, sleep remained elusive. Probably he just had to get used to this new place. With a feeling he had been laying awake for hours, he stood up from his bed and looked at the clock.

"Five past eleven?!" he exclaimed. "Impossible! I went to bed at 10! I thought it was at least around midnight…"

He sighed, and fell down on the couch. "I need some sleep, or I will really fall asleep tomorrow." Cassius muttered. Then an idea struck him, and he rose on his feet. "I hope Professor Snape is still awake."

He silently left his room. "Lumos," he whispered, and after some searching he found a map of the castle in his case.  He fixed where he was, and where he wanted to be, and then walked down to the dungeons, with his wand stretched forward, sometimes looking on the map.

When he had only one more corridor to walk through, he put away the map. Carefully Cassius knocked on the massive door. There was no answer. When there was also no response after the second knocking, he put his hands on the door, and leaned towards it. The door was too massive to hear anything through. Cassius startled when he felt himself falling forward, as the door opened. He swallowed, looked around once, and then went in.

The place wasn't to call 'warm', in fact it was freezing, and everything was dark, except for the top of Cassius' wand. He walked a bit, and studied the various potions. At last he arrived at the blackboard, which seemed rather new. But the dust on top of it made clear that it wasn't new, it was just.. Barely used. Cassius saw another door behind the blackboard and he suddenly burned of curiosity. He was about to enter, when he was startled by somebody clearing his throat. Cassius quickly turned around, to see Severus Snape standing there.

"I see you are awake enough to sneak into my dungeons, Professor Ingaster."

"I wasn't sneaking into your dungeons!" Cassius exclaimed, surprised that the man was jumping to conclusions so soon. "I only wanted a sleeping potion, but you weren't here, and the door was unlocked, so-"

"So you thought you would just go in and a find a sleeping potion in somebody else's room?"

Cassius hesitated. He did make it sound like it was a crime. "Uh, yes."

Severus' eyes narrowed, and he turned to a huge case, and picked out a bottle. "One draught will make you sleep until morning."

Cassius took the bottle, thanked the Potions master, and wanted to leave. When he walked past Severus, the scent of blood penetrated on him, and he roughly turned around. They stared at each other, both of their gazes full of suspicion.

"Good night," Severus said emphatically, and Cassius also found it time to leave.

"Wow," he said to himself as he walked back to his room, "I'll eat my map if that guy isn't evil. Hopefully the potion is a real sleeping potion, and not a poisoning one."

*~*~*~*

"My name is Cassius Ingaster, and I hope I can last here longer than your previous DADA teachers. I-"

A girl with brown hair and a pile of books and notes on her desk, put up her hand. Cassius was surprised there was a question already.

"Yes?" he asked.

"Isn't Cassius Latin for 'vain'?"

"That's right. I didn't know you had Latin here at school too. Five points for Gryffindor." Cassius wanted to start today's subject, but the girl continued.

"Well, we haven't got any Latin, but Latin is a very interesting language, because it returns everywhere. So when I'm not working ahead, I try to get up with Latin, and I've just read this excellent book about the history of the Romans-"

"Thank you, I get the point," Cassius interrupted her, afraid she would be talking for the rest of the hour. "What's your name?"

"Hermione Granger, sir."

"Okay Hermione, could you step forward? I'd like to demonstrate a few things. And you too, please." He pointed at a boy with black hair and glasses. When he saw the scar, Cassius realised this was Harry Potter.

Both stood up, and walked to the blackboard. Cassius let the two fight a short duel, to find out what spells they already knew, and he taught them some new. The rest of the lesson they spent practicing the convisibility spell, which made you invisible for a short time, enough time to surprise the enemy with a spell from an other direction.

Later that day, when Cassius was about to teach the Huffepuffs the same spell, the door suddenly flew open, and Professor Flitwick stood there, sweat running down his face.

"Professor Ingaster, the Headmaster asks you to come!" the small professor squeaked. Cassius opened his mouth to say something, but shut it. He turned his head to the students, who were talking quietly.

"Read through page 25 to 39 and answer the questions on pages 40 and 41." He stood up from his desk, and followed Flitwick, who was nervously wringing his hands.

"What has happened?" he asked.

"Emergency! Death eaters are taking some halfbloods hostage!"

"What?!" Cassius exclaimed shocked. "How did they come in? Where are they? How are the hostages?"

Flitwick shrugged his shoulders. "I don't know!"

As they ran into the Great Hall, they saw many of the staff were involved in a tough fight with some Death Eaters. All of the Death Eaters were masked, and their bodies were covered in black robes. Those of them who weren't fighting, were keeping an eye on the students who were bound on the wall. All together there were 14 Death Eaters, Cassius counted. Noise could be heard from all corners of the Great Hall; it was a mix of frightened screaming, dark laughter, the shouting of spells and curses, and the groaning when somebody was hit. Cassius saw Dumbledore on the other side of the hall, and the fire in his eyes could even be seen from a distance.

Cassius tightened the grip around his wand, and pointed it menacingly at a Death Eater who was about to cast a (probably very nasty) spell on a colleague, who was already on the ground.

"I wouldn't do that if I were you," Cassius said, grateful of the calmness in his voice. As a response the Death Eater directed his wand at Cassius. Before he could react, the black figure had cursed him.

"Crucio," was the last thing Cassius heard before he felt pain racing through all his bones, bones that he hadn't known of, until now. He let himself fall on the ground, hoping the pain would ease there a bit, but it didn't. Every touch with the floor made him feel like his skin was burning, but trying to get up was a hopeless task. He closed his eyes, squeezed his nails into his palms and screamed in an attempt to scream the pain out of his body, but nothing helped. Through all the noise he could hear a familiar voice shouting:

"You! What are you doing?" It was a familiar voice, but Cassius couldn't place where he knew it from. As sudden as the pain had come, it disappeared. When Cassius opened his eyes again, he saw another Death Eater approaching the one who had cursed Cassius.

"We've got more important things to do," he said, and pulled the other away. Cassius inquisitively stared at the tall death eater, and then it dawned on him.

"Severus!" he gasped when he and the other returned to the group of Death Eaters.

Still feeling like he had just fallen from a great height, Cassius stood up to see more wizards arrive. The Death Eaters were now on the losing side, and suddenly there was a loud noise and a very sharp light, which made Cassius fall on the ground again, desperately trying to keep his eyes from burning away. When the light had faded away, there were no more Death Eaters around. Some teachers went out to ensure the Death Eaters had in fact left, but Dumbledore told them not to.

"They're already gone, thank Merlin. We are very lucky to still be alive. And to have all of our students unharmed. God knows why they left them here."

Some wizards objected, but the Headmaster insisted. The students were not hurt, only terribly shocked, if not traumatized.

When Dumbledore went to his office to arrange things, Cassius stopped him.

"Professor, I need to talk to you. It's urgent."

Dumbledore looked at him for a moment, then nodded, and walked to the office. When they had arrived, they both took a seat.

The portraits of the past headmasters which had once looked so cheerful, now all had a serious, grave look in their eyes, and on their faces, just like the headmaster sitting in front of Cassius. His brows were furrowed deeper now than ever, which made Cassius shiver. Fawkes silently sat on top of its cage, and if he hadn't known any better, Cassius would have sworn it was a sculpture instead of a living phoenix.

When Cassius went to start, there was a knock on the door.

"Come in, Severus," Dumbledore said, and the door opened. Severus looked suspiciously from Cassius to Dumbledore, and then closed the door behind.

"I assume that your message is concerning Professor Snape?" Dumbledore asked Cassius, who nodded puzzled.

"I-" he started hesitantly, "I thought.. You know he is a Death Eater?"

It was Dumbledore's turn to nod.

"And you're okay with it??" Cassius stood up, his hands grabbing his head in an attempt to help himself understand.

Dumbledore remained silent.

"What? You're not part of some sort of plot, are you?!"

Severus let out a noise of annoyance, and to Cassius' terror Dumbledore nodded again.

"You'd better explain it to him, before he starts gossip.." Severus glanced at Cassius. Dumbledore nodded for the third time.

"Severus is an official spy for the Ministry. He passes through information. Only a few people know about it, and normally I would cast obliterate on you, but I trust you will not tell anyone about this."

"Oh," Cassius said daftly, after a short moment of silence. Questions popped into his head, and he thought he'd better ask them now. "Why Professor Snape?"

"He was the best candidate." Headmaster simply answered. It wasn't the complete answer Cassius had hoped for. Severus stepped forward.

"Now, if you're finished, I'd like to discuss some things with the Headmaster." he said, folding his arms and staring at Cassius.

"Ah.. Sure." Cassius wasn't finished, but he stood up, and left the office. At the end of the corridor he stopped, sighed, and leaned against the wall.

Although he felt he had been standing there for some time, fifteen minutes had barely passed when he heard heavy footsteps. He quickly stood up again, and saw Severus approaching. He stopped when he saw Cassius.

"What?" he asked icily.

Cassius nervously shuffled his feet, and stared at the wall opposite him.

"I apologise." When there was no response, Cassius looked up. Severus looked really infuriated, and Cassius could now also see his yellow teeth. "For judging you on what I saw." He immediately regretted his words. "Not that you look evil, of course. You must be very loyal if you're ready to undertake such a dangerous task."

"Yes," Snape said in a low voice. "Dumbledore is a very trusting person. I'm afraid that that can be both a good and bad quality."

Before Cassius could ask what he meant, Severus walked past him.