The room was almost completely empty, aside from a single cage in the center. Inside it sat a very confused, completely ordinary rat.
Julian made sure he hadn't accidentally pulled in Peter Pettigrew, because that would have been deeply awkward. His plan to free Sirius early depended heavily on that rat staying unaware until it was far too late to react. Technically, the only reason Sirius was in Azkaban was for blowing up a street and "killing" Peter along with several muggles. If the rat could be proven alive, then Sirius would have already served the sentence, and then some, for the crime.
...
Once Julian confirmed the rat wasn't Peter, he transfigured his robes into a hazmat suit with an air filter and a face shield.
"FUMOS TOXICUS!" he cast, then stopped mid-action when his magical senses warned him the spell was about to backfire.
Hmm. Either the wand movement or the word choice is off, he reasoned, expression tightening.
He tried again with different wand movements while keeping the same incantation.
Same result.
So it's the word choice. Maybe "toxic smoke" is too on the nose, he thought, frowning.
...
"HYDRAE FUMICANT!"
This time the spell activated, but it clearly struggled to form properly. He canceled it and adjusted the wandwork, trying again and again until he found the movement that actually held.
With his wand aimed at the cage, he cast the spell.
Dark purple smoke, threaded with green streaks, spilled from the tip of his wand and crept across the floor toward the cage in a slow, hungry roll.
The rat watched the smoke warily as it approached.
Then the smoke touched it.
...
"EEEEEEEEEEEEEE!"
The rat screamed the instant contact was made. Its flesh decayed rapidly, sloughing away in pieces. In moments, the sound cut off, leaving only silence and bones in the cage.
Julian cut the spell, his back slick with sweat.
It exceeded his expectations by a wide margin. There was no doubt at all that it would be considered dark magic, purely for the inhumane way it killed anything it touched.
This is absolutely going on the "break out only in emergencies" list, he thought, sickened. Dear god, that poor rat.
...
Julian didn't like anything that caused unnecessary suffering, and if the rat's agony was any indication, this spell very much qualified. Even so, he needed to be able to defend himself. Despite his distaste, he kept the spell in his arsenal.
Once he finished with his attempt to modify the fumos charm for combat use, the room visibly shifted. The space reshaped itself into another empty chamber, except for a bookshelf standing in the center.
Julian approached it while dispelling the transfiguration on his robes.
The books were on shield spells, and he wanted to learn them here, because searching the library when the room could pull non-enchanted books was pointless.
...
That was also why Julian had simply stolen a few books rather than copying them.
Enchantment.
Most of the library's books were charmed to be uncopiable through magic, and writing them out by hand was too time-consuming to be worth it. Some books, however, had the enchantment removed, or the charm had faded over time, making them copyable again.
The room took advantage of that, creating its own copies for use.
...
The shelf held only five books, which wasn't particularly surprising considering how well Hogwarts treated its collection. The librarian regularly renewed the library's self-cleaning and damage-resistance enchantments, once a year.
It meant you usually had to actively try to destroy a book. Doing it by accident was difficult, at least most of the time.
Those five books covered basic defensive charms like Protego, and even the highly advanced murus scutorum spell, which summoned a multilayered dome of barriers around the caster.
...
Julian read every book from cover to cover before deciding what to practice.
From them, he learned that most shield spells were created for personal protection, often by cowards. People terrified of harm but unwilling to learn how to truly defend themselves instead poured their effort into shields meant to repel outside attacks.
Julian found it funny in a bleak sort of way, how cowards had created spells that brave men now used to fight without fear.
He couldn't really judge them, though. Everyone had something they feared, and the fact that they had done something, anything, to confront those fears was impressive enough in his opinion.