This is an eventual cross between Justice League Dark and HP. Enjoy!
Everything was going black. Everywhere, the fear demons that the Ministry mistakenly called "dementors" surrounded her. There had to be hundreds of the things, and worse, the adults honestly believed them to be under their control.
The only reason they humored the humans was due to the fact they were able to feast on the souls more or less unhindered by those who knew how to deal with the damn things.
She could sense them feeding off her. Feeding off the pain and misery in her memories. They were too bold, and the ground was coming up too fast.
She had limited time to act. She knew Dumbledore would stand by and do nothing...hell, he had been absolutely useless with the possession case last year!
It was past time to call in the big guns, if the demons had become this bold.
It took everything in her power to begin the incantation, and then desperation took hold. The demons began to swarm in greater force, sensing it. They knew she was doing something.
Her eyes glowed as the spell she had seen once in her uncle's massive library began to take effect.
Uncle John...HELP!
She blacked out, as her soul was swapped with another.
"She" was saved before she hit the ground at speed, but the teachers had no idea what she had unleashed in their school.
Let him deal with Dumbledore's nonsense...it was time someone with actual experience straightened out that mess! It was only supposed to last a week anyway.
Several hours later...
The small frame in the bed took stock of their situation. It was clear that Circe had felt desperate enough to cast a rather dangerous spell to bring him to this place, and he could see why. There were a ridiculous number of fear demons festering about with the wards barely keeping them in check from the hundreds of innocents within. Why they hadn't drained the town outside of their souls was anyone's guess though.
He sifted through the more recent memories and almost winced. No wonder she had called on him...the headmaster was a bloody idiot for allowing these thing so close to a school.
Fortunately he had been around her enough to fake her personality, though Dumbledore clearly suspected something.
He had to hide a smirk when he saw an old flame of his, though they had parted under difficult circumstances. Remus never did like the risks he took, but he had been deep in sentimentality when he had hooked up with John.
Ditching her friends was easy enough. And a quick search of her trunk revealed the ingredients he'd need to cast a few spells.
First things first though.
John spoke in an undecipherable tongue, casting a spell in the hidden room Circe had prepared. He saw the relieved expression on 'his' face on the other end. He had trained her well.
"Uncle John!" said Circe. It was weird seeing his body from the other side.
"Hello love. I can see why you swapped with me, place is festering with the bloody things," said John.
"I wasn't able to prepare a proper spell to banish them, since a lot of the ingredients are too sketchy for a thirteen year old to acquire, especially this close," said Circe.
"Good choice, love," said John. "A bit weird being in your body, but I've seen worse attempts at spells like this."
Circe beamed at him.
"Watch out for Snape and Dumbledore. I suspect they know something, especially after last year."
"Not the first time I've dealt with tossers like them. For now, stay in the house and don't leave, at least until the spell wears off."
"Understood, Uncle John. Take care," said Circe.
John let the spell drop, and used the cloak Circe had brought home two years ago to hide from the teachers who were racing down the corner. Apparently someone had sensed his spell and had come to investigate.
Too bad for them, he was better at sneaking around than they were. A pity about her broom, but he'd be sure to get her a new one later. Something better than whatever these wizards could create.
It was never too early to learn how to drive a bike, after all.
A lone figure walked outside the castle. The demons salivated over how dark the soul was.
"Bloody hell, wish I had a cig," said a voice, mostly quiet. Moonlight revealed a young woman of about twelve to thirteen years old, dressed in a cloak. She pulled out a bottle of firewhiskey and took a long swig. "At least those twins were good on their word."
Fred and George were among the first to notice it wasn't Circe walking around in her body, though that was mostly through a cheat.
A simple explanation (and another call to the house) had things sorted in short order.
If it meant protecting Circe and the others from the dementors, then they'd gladly source whatever 'she' needed.
The dementors began to swarm.
"Oh no you don't, you bloody bastards. You've had enough fun as it is. Time for you to go back to where you came from," said 'Circe'.
Sensing danger, more dementors began to swarm closer to the girl. 'Her' eyes glowed as she began a long, indecipherable chant in an arcane language that hadn't seen proper use in many centuries.
Circles made of hell fire formed from her hands.
She tossed something in front of her, where the largest cluster of demons were.
Their shrieks would have woke the dead...and by now most of the students and teachers should have been rushing to see what was going on.
Or they would have, had 'Circe' not had a talk with the house elves to insure everyone was sound asleep thanks to a few potions.
The twins were watching just to be safe, in case 'Circe' needed to be pulled back behind the wards.
They were watching with wide-eyes as their adopted 'little sister' began destroying demons left and right.
"Bloody hell," said George.
"I know she said her uncle was powerful, but I don't think she mentioned this!" said Fred.
The two looked at each other with manic grins as they said in unison.
"Bloody wicked!"
'Circe' knew the Ministry would have words with why so many of their pet demons were now gone, but she couldn't give less of a damn. These things nearly killed the only blood family 'she' had. They didn't deserve any mercy.
Seeing what looked like a church grim staring at the scene mouth agape in shock and disbelief, she smirked.
"Are you going to skulk about forever, or are you going to piss off?" she called out. "I have no issue taking you out with this lot."
The grim shook it's head in shock, before it transformed into a familiar figure.
"Sirius bloody Black. Why am I not surprised," said 'Circe'.
"You are not my pup. Who the hell are you?" he growled.
"John Constantine, Master of the Mystic Arts and all around wanker," 'she' said with a mocking bow.
Sirius stared at 'Circe' hard, his jaw dropping in disbelief.
"How the bloody hell did you pull this off, and why a thirteen year old witch?"
"Oh, don't look at me like that you bloody tosser," said John, glaring at him. "I'm not the one who cast this particular spell...she did. Besides, do I need to remind you of what you pulled in Sussex?"
"That was your fault and you bloody well know it!" said Sirius, his face red with embarrassment.
"Sure it was," said John mockingly.
Sirius knew John would at least hear him out, rather than curse first and ask questions never.
"So what are you doing here John?" said Sirius.
"Like I said, Circe called me here. Seems she felt that the headmaster's protections were sorely lacking, after those demons nearly did her in."
"She's not the only one," said Sirius growling.
"And what about you? Last I heard you were thrown into their feeding ground in the deepest pit they could find."
"Peter betrayed us. Told the dark tosser where to find Lily and James, then framed me. I only came back because I saw him in the newspaper, with a known pureblood family. Couldn't stand the idea of that bastard being so close to Circe."
John narrowed his eyes at Sirius. The mutt was many things, but when it came to James he would never lie about something like this. Besides, he always thought something was off about the whole story, since he knew damn well that Sirius would have likely taken the magical oaths to be Circe's godfather.
"So, what form did the bugger take?"
"A rat. A common rat," said Sirius.
John paused and stared at him.
"Wait. That bloody rat Scabbers has been living in the same dorm with my niece?" growled John.
"Your niece?!" said Sirius.
"Petunia called and asked me to take Circe in. I knew I couldn't raise her, not with all the people after me, so I left her with a friend and kept in touch," said John shrugging.
"Smart move," said Sirius, as there was no way he could see John being a responsible parent.
"So what's the plan, go in, kill the wanker and be thrown back in Azkaban?" said John.
Dead silence.
"You're shitting me. That's the best you could come up with?" said John incredulous.
"Look smart ass, you have any better ideas?"
"I'm stuck in her body for the next few days. Might as well have some fun with the old codger and the cat," smirked John. "For now, leave Peter to me. I suggest making yourself scarce for a while."
"And where the bloody hell am I supposed to go?" said Sirius.
"The House of Mystery. You can hide out there until this gets sorted, but you'll do no one any favors by sticking around here. Especially since those morons at the Ministry thought it was a brilliant idea to stick fear demons near fresh targets."
"And how am I supposed to find that?" said Sirius. John took it as a good sign that he was openly considering it.
"You don't find it. It finds you," said John. "Besides, Circe is there in my body...for now."
In the House of Mystery...
"WHY IS THERE NO FOOD?! Bloody hell, does Uncle John only subsist on booze?!"
Back at Hogwarts
John made quick work of the demons around the school. He was able to put up a rudimentary ward around the place to keep them as far away from the common areas as possible. Why Dumbledore allowed their presence was a mystery to him.
Sirius stuck close, and made sure to guard his pup's body. He was still a bit confused, but at this point he knew who's side he was on. John might be an utter bastard, but if he had to choose between hellspawn and John bloody Constantine, he'd pick John every time.
"Sun's coming up. Best you bugger off mate," said John. Sirius barked in agreement, before bounding off.
John looked at the sunrise, and really wished for a cigarette. At least he had some semi-decent booze to drink.
The twins carried Circe's body back to the common room, and put her on a couch with a blanket. Circe had a bad habit of sleeping there some nights, when she was deep into a good book, so no one would look twice at the scene.
By noon, everyone was beginning to wake up, and 'Circe' had about six hours of proper sleep.
Needless to say the teachers were less than pleased and began looking for the culprits of the mass potioning. And that was before they realized they were missing quite a few dementors.
The Ministry was beyond livid, and already organizing a massive hunt for where their pet demons had disappeared to.
'Circe' merely yawned and poured a stronger than normal cup of coffee to try and wake up.