Having experienced it twice before, Sherlock was already incredibly familiar with the reaction of the mark on his left arm.
But familiarity did not mean getting used to it.
On the contrary, the sudden burning sensation in the library made him freeze.
The mark had reacted twice before: once when he encountered the Malfoy father and son at Flourish and Blotts, and another time when he bumped into Neville on the Hogwarts grounds.
Now he was in the Restricted Section, a place generally forbidden to students, with no one around but countless forbidden books lining the shelves.
This time, the burning sensation on his left arm did not stop abruptly; instead, it continued, contrary to its usual behavior.
This peculiar reaction made Sherlock frown.
He took a few steps back, exiting the Restricted Section and returning to the regular library area where students studied. At the same time, the burning sensation on his left arm quietly disappeared.
Then he moved forward again, re-entering the Restricted Section, and the burning sensation on the mark returned. It felt as if the mark were using this method to hint at something.
Sherlock fell into deep thought.
He covered his arm and began walking along the first row of bookshelves in the Restricted Section. The closer he got to the interior, the stronger the burning sensation on the mark became, until it reached its peak when he arrived at the sixth-to-last bookshelf.
Continuing further back, the sensation began to weaken instead.
Noticing this, Sherlock became even more certain that his guess was correct: the mark on his arm was trying to convey a message, and the source of that message was on this very bookshelf.
He stood before the sixth-to-last shelf of forbidden books, extending his left hand to gently trace the first row.
The burning sensation persisted.
He continued to trace the second row.
Still no change.
Just as his finger touched the first book in the third row, a slight, needle-like prickling sensation suddenly shot through his left arm.
Immediately, the burning sensation receded, as if it had never reacted at all.
But Sherlock already understood: this was the book it wanted him to take.
He gently pulled the thin forbidden magic book from the shelf. Under the nearby light, Sherlock saw the book's title.
'Soul Stitching'
The title was handwritten in black ink, and judging by the letterforms, it had not been written too long ago.
The entire book was very thin—roughly only a dozen pages. Calling it a book, it was more like a notebook.
The texture was somewhat similar to parchment, but noticeably thinner.
Sherlock stared at the strange title for a moment, not opening it immediately. Instead, he walked over to Madam Pince and registered it for borrowing.
While registering, he subtly observed Madam Pince's expression.
She showed no obvious change from beginning to end, as if Sherlock were merely borrowing an ordinary forbidden book.
From the book's cover to Madam Pince's reaction, everything about it seemed unremarkable.
But Sherlock, already walking out of the library with it, knew very well that anything capable of making the mark on his left arm react was definitely not ordinary.
Back in his office, he locked the door from the inside. Then, holding the book, he sat down at his desk and placed it face-up before him.
In the Wizarding World, virtually all adult wizards warned their children with one phrase:
Do not easily open magic books, boxes, or bottles of unknown origin!
This warning was as common as parents in the normal world reminding their children to look both ways before crossing the road.
Even Sherlock naturally knew the correct way to handle a book of unknown origin.
Drawing his wand, he first gently tapped it on the book cover to check for traces of magic.
Then he aimed his wand at the book and cast Alohomora.
After a gentle breeze passed, nothing changed.
For the final step, he took a step back, standing a meter away from the desk, and slightly raised his wand.
The first page of the book opened, with no unexpected events.
From a distance, Sherlock saw the title page, which was completely blank, without even a trace of ink.
His wand rose again, and the blank page turned. On the first page, he finally saw the book's contents.
Even though no surprises had occurred yet, he still did not move forward, remaining a meter away as he began to read.
'When I narrowly escaped from Norca Town, I noticed that my condition was off.'
'The prolonged Cruciatus Curse, though it did not drive me mad, still caused damage to my soul.'
'Of course, at first I merely suspected, but after a series of self-examinations, I confirmed my current situation.'
'My soul was torn.'
'The effects of this tearing are not usually visible, but the soul is crucial to magic. Because of this damage, my magical proficiency deviated, and the power of my spells was greatly diminished.'
'I knew I had to find a way to solve this problem. My enemies would come for me again, and I had to ensure my strength did not decline.'
'The first method that came to mind was Potions. As the most commonly used healing method for wizards, perhaps among millions of formulas there was one capable of healing the soul.'
'I wrote to my teacher, but he told me that no potion had ever been able to affect a wizard's soul. Potions act only on the physical body, while magic is a reflection of the soul.'
'Still, my teacher gave me a direction: magic is a reflection of the soul—especially ancient magic.'
'I scoured nearly every ancient magic book I could find. There were countless records concerning the soul, but not a single one about healing it.'
'Ha! The wisdom of our predecessors is nothing more than this. Since no such magic has existed from ancient times until now, I shall create it myself.'
'Healing the soul is extremely troublesome, but thankfully, like the body, it can self-repair—though this process is very slow.'
'Especially when torn, the difficulty of recovery multiplies.'
'Just as I was at an impasse, a Muggle surgical procedure inspired me.'
'When Muggles suffer severe wounds, they use sutures to reconnect the flesh, accelerating healing.'
'Seemingly crude, yet in truth a brilliant idea.'
'Since the soul and body are interconnected, both capable of self-repair, could I not also stitch my torn soul together, as Muggles suture wounds, to accelerate its recovery?'