Hello again!
One more sequel of sorts to Changes, though it's really a sequel to Light, if anything. Set in the same world. The Doctor has visited Hogwarts previously.
This chapter pretty much just sets things up. There will be twists and turns, (what else would you expect from me? :p) as well as a few moments I just couldn't resist adding. one such moment is in this chapter.
This is just the start. Enjoy!

"I am sorry to say," Dumbledore's voice boomed out over the Great Hall, "That this year, Hogwarts will play host to the Dementors of Azkaban."

Silence greeted his words.

At the same time as Dumbledore delivered his speech to the hall of students, one of the forbidding, black-cloaked figures was gliding through the rain, lifeless skin just peering out from the coarse, dark fabric.

One of many. A group, a swarm of black abominations, darker than even the thick cloud cover which pelted the rain down. Happiness, heat and life was drained from the grass and land around them; death walked in their path.

Shadow garbed the Dementor; its presence brought frost to the grass, and the huge swarm brought clouds. And darkness descended, as if it was as good a cloak as the black the fiend already wore.

The Dementor glided onwards, over frosted grass, lean, predatory, dark.

And it screamed.

O

Rowena of Ravenclaw could remember the odd visitor to Hogwarts the last year. The Doctor, he'd been called. An investigator; called to examine the oddities plaguing the school.

She was in her last year now; which wasn't shaping up well. Dementors in Hogwarts, to catch that prisoner. Sirius Black. Still, she'd get through it. Hopefully.

Yet her thoughts kept drifting back to that investigator. The Doctor. Last year, things had felt a little off; just…irregular almost, incorrect. Something unnatural had been going on; especially if you counted the Chamber of Secrets, and the centaur, and the rocket within the Forest.

This year, there was the same feeling; an instinctive wrongness. Everyone felt it, they just didn't admit it.

Rowena was acting on it.

It was dark; but really not that late. The Dementors had made the night come sooner. Even the natural world was affected by their presence; it made the seventh year shiver.

She tentatively walked through the courtyard. Alone. Abandoned. Most people were holed up, at the slight risk of a Dementor attack; Rowena was fighting off that fear as she moved. Her intended destination was the Doctor's room; the classroom he'd converted. While the investigator was no longer here, maybe there was a way to contact him.

A whoosh of black.

Breath condensing on the air, Rowena turned sideways, sudden, jerky, afraid. A Dementor was just there; steps away. Yet, strangely, she wasn't depressed. Well, not as much as she should be.

The girl in her last year frowned, taking a step back from the Dementor.

The fiend's black cloak turned; its body was unseen, though the cloak made her aware of each movement. Something stared directly at her from behind the sable hood.

The Dementor turned entirely, facing Rowena completely, sides of the cloak billowing in an unknown wind. It glared with empty eyes.

Rowena was oddly reminded of the Grim Reaper, that Muggle fairy tale. Black hood, and a cloak of the purest, deepest black.

She fumbled for her wand, inexplicably worried. It shouldn't be scaring her; it shouldn't harm her. It shouldn't. But it didn't feel anything like a normal Dementor; fear raced through her system, erasing all signs of depression.

Silent, the Dementor glided forwards, cloak spread wide as it rushed Rowena. Darkness emanated from it, consuming, devouring.

The fiend reached her, cloak snaking over her body.

A scream.

O

Divination. Not the most fun lesson, decidedly.

Harry wasn't all that glad he'd taken it for this year, but nonetheless, he was here. And he wasn't quite as annoyed about it as Hermione; who had somehow entered the lesson without them noticing.

In the centre of the class was their teacher, Professor Trelawney, a slightly crazy, perpetually wide-eyed woman.

"Welcome," her misty voice carried through the room.

As her vacant introduction was related, the students did their best not to look out the window, not to be distracted by the driving rain. Even after Dumbledore's pronouncement, some Dementors entered the forbidden grounds of Hogwarts; one such fiend was visible out the window. Some wizards could be seen throwing animals of light at it, driving it away.

Concentrating through the drowse-inducing incense, Harry, Ron and Hermione swallowed a cup of scalding tea, beginning the first lesson in tea-leaves reading. Not the most stimulating skill, but they focused on it with as much effort as they could, despite the aura of the room, which seemed to sap their concentration.

"That's a…" Ron paused, frowning at Harry's cup, "Looks like a bell," the redhead consulted his copy of the textbook, "So apparently you're going to get some unexpected news. And that's a boat…so you're going to meet a friend. Well, I'm here, so that one's happened. And that's a finger…" Ron frowned

"Emphasizes whatever it points it," Harry supplied, flicking through his book, "Which is…that," the black haired boy gestured to a sodden mass of tea leaves, the last piece in his cup.

"I think that's…a table? Or is it scissors. Well, it's either a club or a quarrel coming for ya. Good luck. As long as it's nothing like Lockhart's mess of a club last year."

"Mm," Harry nodded, remembering the fiasco, before looking at Ron's cup. "Well, that's a mess, or an octopus, which means danger, but that's a harp. Harmony. So you're in danger, but it's going to be calm. And flirtatious judging by that fan."

"At least I'm not quarrelling with a finger," Ron retorted.

The muffled their laughter as Trelawney neared. The wispy seer seemed drawn to the duo; and after a few seconds, mildly rudely, had snatched away Harry's cup.

"Ah, my boy," Trelawney seemed to exhale excessively as she spoke, sounding more like a gust of wind than a teacher, "The goat…" she kept rotating the cup, "My dear, you must be careful of your enemies-"

"Obviously," Hermione surprised everyone by interrupting, rolling her eyes.

"Excuse me?" Trelawney semi-snapped, yanked out of her reverie

"I think everyone needs to be careful of their enemies," she stated, so that the entire class could hear.

Harry and Ron stared at her, in a bizarre mix of admiration and surprise. It wasn't often that she spoke back to teachers; especially in that fashion.

"Yes, yes," Trelawney hastily moved on, "And this…" she peered at the leaves Ron identified as a table. "It is…"

The teachers topped her eerie speech, dropping the glass with a quick gasp. Somewhere over the other side of the room, there was a tinkle as Neville broke his cup.

"This is not a happy cup," she murmured. "It would be kinder not to say."

"What is it, miss?" Lavender asked keenly.

Indeed, most of the class was listening in; thoroughly absorbed by her monologue.

"Very well," the divination teacher slowly spoke, staring with wide eyes, "It is…the Grim."

The class inhaled as one; a moment spoiled somewhat by Dean Thomas: "What's a Grim?"

"My dear," Trelawney stared, "The Grim is an omen, the worst omen, of death."

Before anyone could react, a voice was heard, booming throughout the castle. Everyone stiffened; listening intently. As the speech continued, Trelawney seemed a little disappointed, as well as tense. Most of the class shared that tension.

The voice belonged to Dumbledore: he was not present, instead he magically amplified his voice, to be heard throughout the entirety of Hogwarts: "All students to return to their Common Rooms. All students, class is dismissed, return immediately to your Common Rooms. Prefects and teachers to the West Courtyard."

There was a rustle of fumbling and packing, before the class quickly made their way out of the room. The combination of Dementors, and the urgency in the headmaster's tone, was incredible motivation.

Outside the window, Harry noted with interest, a large group of unidentifiable wizards, teachers mostly, held a Dementor at bay with bright, white light from their wands. However, it took a shining white phoenix, borne of magic and sent from somewhere in Hogwarts, before it was truly repelled.

O

Several minutes later, each and every Prefect in Hogwarts was gathered in the courtyard, a chaotic rabble despite their standing. The teachers were also there, and they weren't behaving much better; babbling among themselves.

"Silence!" Dumbledore boomed, wand at his throat to amplify the noise.

The command instantly made the crowd hush. Nervous, the whole group looked towards the tree in the centre of the courtyard; where the headmaster stood.

Dumbledore did not speak. Instead, wordlessly, he pointed with his wand to something by the wall; it had been covered with a blanket formed by magic. The headmaster withdrew the covering, flicking it into the air, and dissolving it into a series of sparks.

Beneath it was a black, damp, tattered cloak. A sheet of fabric; easily recognizable. It was one Dementors wore. Yet there was no sign of the fiend which normally inhabited it.

Still silent, Dumbledore levitated the cloak. Beneath it…

More, frantic, scared murmurings began in the crowd. Prefects looked at each other, teachers muttered amongst themselves, cautious, preparing, but most of all, afraid.

"This is why I have summoned you," Dumbledore was forced to magnify his voice to be heard over the driving rain, and the babble of voices; "Such a situation has not occurred before, and I am not alone in thinking we need help."

A pause. The audience frowned.

"Last year, as some of you may recall, we were visited by a man who seemed wiser than many of us, myself included. He saved us then, even if you are not aware of it. His name was the Doctor."

Remus Lupin, the new Defence Against the Dark Arts teacher, raised his eyebrows at that name, turning to the headmaster, much more interested.

"Have you heard of him, Remus?" Dumbledore lowered his volume, speaking curiously to Lupin, as if to a friend

"By reputation," the teacher conceded, nodding, "There was a woman when I attended Hogwarts, River Song. She spoke of him; she said to trust him, if we ever met."

Dumbledore nodded, interested by the new piece of information. However, he still continued speaking. The Prefects and minor teachers watched.

"The Heads of Houses and I have come to the conclusion that the Doctor is again needed. And so I have called you, to be witness. While I am against anything so abrupt, I feel there is no choice."

Albus Dumbledore, headmaster of Hogwarts, took a stepped forward. A little distance away, the lost cloak of a Dementor rippled, over something else. Rain poured down over them, soaking the cloak and Dumbledore through.

Dumbledore raised his wand high, before proclaiming in a clear, loud voice:

"Accio Doctor!"