At the end of the lecture, Harry urged his friends to head off without him, claiming he had a question for their stuttering professor. Confused but not academically driven enough to stay and listen to more material, Ron happily dragged Hermione off with a promise to save him a seat at dinner.
Professor Quirrell didn't look up from where he was grading papers at his desk. Heavy darkness swirled around his head, the shadows bursting from his skull - an obvious sign of possession. Harry was a bit embarrassed to admit he hadn't realized the severity of the situation immediately - he'd just thought the man had been snacked on by a vampire and was shrouded in dark energy as the result. It would explain all the garlic. He was still very confused about the garlic. Did spirits eat garlic? Or was it a red herring?
Anyway, he had been unable to determine what sort of thing was possessing the man. It had to be fairly powerful to take over this much of his soul, so it could be a demon or a wraith. Harry wasn't very concerned either way. He'd helped in a few exorcisms before, and mama had sent over a long list of instructions and tips.
The apprentice rubbed a tattoo hidden underneath his school robes - seven equilateral triangles forming a circle around a rune. It would protect him from possession in case something went wrong - but it would not protect any of the other children, so Harry did have to focus carefully on this task. He couldn't bear the shame of his exorcism going bad and letting a dark shade loose in the castle. Thus, Harry had begrudgingly studied up on the ritual during Quirrell's class.
Taking a deep breath, he let his magic wash over the room. The shadows shuddered and stiffened as he took hold of them. Their ends wrapped around his wrist and coiled and shifted with energy. It was as though he was now holding the leashes of several angry dogs ready to strike. As the room steadily grew darker, Harry pulled out his wand and silently cast to summon a vial of pure salt. A shadow curled around it and laid down a salt circle around the professor's desk. At that moment, Quirrell noticed the darkness as he drew the papers closer to his face to see clearer. Confused, he noticed the silent figure at the door of the classroom. Knowing the spirit was trapped only as long as the professor didn't break the circle, Harry began to chant the spell to remove the possessing being.
Lord Voldemort was not an idiot of course, and immediately recognized the spell and began to panic. Well, no one heard him panic except for Quirrell, who was also panicking - but mostly because Voldemort was panicking - he figured if something was enough to startle Voldemort it had to be serious. The Dark Lord ushered his host into a counterspell which the man began to mutter as his shaking hands unwrapped his turban. The panicking had cost them though - neither noticed the swirling shadows or the line of salt completely encasing them inside it.
Harry, meanwhile, was also a bit panicked. The twin faces protruding from one head was a disturbing image; the fact that he was casually exorcising Lord Voldemort was far creepier. What was with this man and residing inside people's heads? Did he not have anything better to do? Harry's incantation faltered only a moment as he quickly moved into action. He had the shadows wrap around the professor like vines, trapping him and keeping him from continuing his own magic.
But Voldemort fought against the shadows with all his might. After a long moment of flailing and cursing, Quirrell released a great scream and collapsed, his body twitching and prone. Voldemort's shade began to ooze from his skin - his flesh stripped away like bubbling tar pushing through Quirrell's pores. His spirit now bounced freely around the perimeter of the circle, wailing and hissing Dark curses as he tried to flee. But the circle would not yield to Voldemort's incorporeal form, and Quirrell was too unconscious to be of much assistance.
That didn't mean Voldemort was defenseless, however. He had been ripped out of Quirrell but the wizard was skilled in Dark magic just as much as Harry was. Although he was trapped, he could still counteract the necromancer's attempt to exorcise him. Harry's magic gripped onto the misty form of the spirit and with a harsh curse began to drag it towards the earth. His hands were raised as though in prayer and his eyes glowed in the wildly flickering light of the room. Shadows arched out of his palms and spread out around him, hissing and howling as they began to latch unto the spirit and drag it into hell. But Voldemort resisted, slipping and sliding through the metaphysical hands grasping at him. The shadows were unable to keep a hold of him.
Desperate, Harry gathered all his strength. He made a scooping gesture with his arm and the room plunged into total darkness. A massive horde of shadows rushed at the spirit and, well, spirited it away - the chaos and power let the horde grip Voldemort enough to push him beyond the Hogwarts wards. It was not a proper exorcism - his shade was still out there - but it was at least weakened and not a direct threat to his fellow students.
Voldemort's screams of pain and anger echoed across the grounds, and Harry suddenly realized he was in a particularly suspicious scenario. The candles flickered back to life, and the boy found himself next to a bloody body. A quick brush over the form with his magic confirmed that Quirrell had died and his soul had been ripped to pieces. Did he have time to do something with the body? Would that make it more or less suspicious, he wondered? It was extremely inconvenient for his professor to die. Did he have time to harvest his bones for divination?
Deep scratches bore into the walls and a fine layer of salt covered the room like dust. No, it would take too long to clean everything up, and ironically enough, while the boy was great at necromancy, his repairo needed work.