The troubles started one afternoon during class.

"If there are no questions, you may proceed," said tiny Professor Flitwick, the Charms teacher. "Remember the long full sweep of the arm!"

The students lifted their wands and began to practise.

"Mobililibrus," Ron said hopefully, pointing at his textbook, and to his delight it lifted off gently from the tabletop and swayed in the air.

"Mobilisellus," Harry said firmly to an empty chair, raising it in the air and moving it to the other side of the class. He grinned at his success.

Dean Thomas winked at Harry. "An empty chair? Child's play. I'll top you. Mobilisellus!" he said, pointing to the chair occupied by Neville Longbottom, who found himself floating three feet off the floor.

But Neville was unperturbed. Sweeping his wand at Dean with surprising grace, he cried, "Mobilibarus!" and Dean rose into the air with Neville.

"Careful now, boys," Professor Flitwick called out.

Dean and Neville set each other down and Neville leaped off his chair so that it wouldn't happen again. Ron, still practising, picked up Neville's chair with another "Mobilisellus!" and tried to carry it towards him, but accidentally flung it against the wall.

"Mr. Weas-" Professor Flitwick began sternly, but was interrupted by a quiet sound.

Creeeaaakkk. It came from the wall Ron had hit. They all turned to stare.

"Professor?" Lavender Brown said nervously.

Crrrreeeaaaakk. It was louder. Threatening.

"Everyone out," whispered Professor Flitwick. Crrrrreeeeeaaak, went the wall. "Now!" he said.

The students grabbed their schoolbags and ran out into the hall, followed closely by Professor Flitwick. Then the wall collapsed, completely crushing the room where they had been sitting moments before.

In a Transfiguration classroom on the other side of the castle, a class of Hufflepuff first-years were learning to change matches into needles.

"Try it now, everyone," Professor McGonagall said to the students.

A girl lifted her wand and pointed it at the match. The match began to lengthen and the girl was delighted. But her smile faded when the match became scaly and continued to grow longer and longer till it was four feet long, and then one end of it twisted round and hissed at the girl, sticking out its forked tongue.

The girl screamed and jumped back from her desk. And through the whole classroom, the same phenomenon was occurring. The matchsticks were transforming into snakes of all types and sizes, all of which slithered off the tables and onto the floor. The first-years ran out screaming, and Professor McGonagall ran after them. At the door she turned and looked back briefly at the classroom writhing with serpents, horrified and shocked, stuggling to comprehend what was happening. Then she slammed the door.

In the dungeons of the castle, Professor Snape paced between the cauldrons of the sixth-year Ravenclaw and Slytherin students. "Do not forget that the chopped daisy roots go in only after the spine of lionfish."

"Professor?" called Cho Chang nervously from the back. "The floor is covered in water back here."

"What appallingly foolish person has been stupid enough to spill water on my floor?" demanded Professor Snape. The students were silent.

Then a Slytherin near the front of the classroom raised his hand. "But Professor, there's water on the floor here too, and none of us have spilled."

"It's already two inches deep here," said a Ravenclaw from the side of the dungeon.

Snape frowned and looked down. He too was standing in water. He raised his eyes and looked round. A layer of water covered the floor of the entire dungeon.

"Professor," a Slytherin student said fearfully, "it's rising."

So it was. In less than a minute the water level had risen several inches. It spilled over the tops of their shoes.

"Everyone out," Professor Snape said quietly. "Put out your fires, drop your ingredients, and get out now. Go upstairs, and wait for me there."

The students did as they were told. They turned off their cauldrons and ran out of the dungeon quickly, sloshing through the water that now came halfway up to their knees. Snape stood alone in the middle of his classroom, the rising water soaking the hem of his black robes. "How." he whispered, uncomprehending. His face suddenly twisted into a weary, crazy grin. "I must have missed the memo," he said aloud, and gave a bark of deranged laughter. Then he turned and ran up the stairs after his students.

Outside in greenhouse four, Professor Sprout was teaching seventh-year Gryffindors. "Here we have the Alihotsy plants. Ingestion of the leaves causes hysteria. I remember once in my fifth year at Hogwarts, someone snuck Alihotsy leaves into the chicken cassoulet as a prank and all of Ravenclaw house was struck down with a mad folly."

They moved on to a fist-sized clump of black vines and tendrils that sat in damp soil in a shaded area of the greenhouse. "And here is a small specimen of the carnivorous Devil's Snare. You needn't fear it now though, I've put a Sleeping Charm on it."

"Then why is it moving?" asked Alicia Spinnet, pointing to a wriggling tendril.

"Sleepwalking?" suggested George Weasley.

"More like sleepgrabbing!" yelled Fred, pulling his arm out of the grasp of a vine. He fired a Severing Charm at the vine, cutting it off; but it continued to grow out again.

"It's growing too fast!" Katie Bell said to Professor Sprout as she used a Severing Charm to cut a root that had ensnared her foot.

"But it shouldn't be growing at all!" Professor Sprout cried, drawing back from a tendril that menacingly wrapped its arm round her neck. "I- I don't understand, it should be asleep!"

"That is clearly not the case!" shouted George, who was dangling in the air, his ankles clenched in a thick vine. He screamed, because he had just remembered what the Devil's Snare did with things it caught: it ate them, disgesting them slowly over several hundred years. Alicia and Fred reached up and grabbed his arms, and a tug-of-war ensued with George in the middle, shouting in panic. The two managed to pull him out of the plant's grip and all three fell backwards on top of more roots and vines that were sprawling across the ground.

"Everyone outside! Run!" shouted Professor Sprout. She and the students all bolted for the exit. The Devil's Snare stretched itself after them, but they slammed the doors shut on the thick black tendrils. Professor Sprout stared through the glass walls at the huge plant, which by now, against all rules of nature and botanical magic, had taken over the entire greenhouse, crushing all the other plants.

Katie Bell screamed and pointed at the castle. "Professor!"

Professor Sprout turned and looked. She too screamed out loud, because hanging in the sky over the castle was a huge cloud of green sparks, forming a skull with a snake coming out of its mouth: the Dark Mark.