At some point Knox had abandoned the desk.
He'd dropped to the floor, circled half the classroom, and padded over to the window, settling a few steps from Regulus.
Deep blue eyes fixed on the world outside. The moon slid free of the clouds and laid a patch of light across the floorboards. He studied it a moment, then pounced.
Nothing under his paw but wood, and a soft slap of sound.
He tilted his head at his own paw, tried once more, came up empty again, and gave it up.
Tail curled around his body, he watched the window in silence, moonlight plating the tips of his fur a faint silver-grey.
No more hissing at Regulus. Whatever had been crawling under a cat's skin all evening was gone, and the human seemed less objectionable now.
Hence the willingness to come this close.
Regulus glanced at the cat by his foot and left it alone.
"That's enough Constant Protego for tonight." He settled back against the sill. "Grind it out in your own time. There's one more thing tonight, and it's the opposite of that one."
Lily's ears pricked up.
"The Potter match. Your own words. You knew how to steer him where you wanted him, but he was too fast, and you couldn't finish what you'd started."
She nodded. That had been the worst of it.
"Fast and accurate are good things. Keep training both, your hands will get quicker and your spells truer. But speed has a ceiling, and there's always someone faster. When you meet them, your whole style stops working."
She tilted her head.
"So you patch the gap somewhere else. You fight with your head. What you lack is knowing sooner. If you can tell an instant earlier that he's about to move, that a spell is about to come, your style catches up with itself."
Her eyes widened. "There's something that does that?"
"There is." He nodded. "Not like Constant Protego. That's magic manipulation, years of it. This is a spell. Learn it and you can use it the same night."
"The same night?" She leaned in, red hair swinging with her. "What spell? Professor Regulus, teach me!"
The corner of his mouth lifted. "Sensus Altus." [Supersensory Charm]
"It governs your senses," he went on. "Eyes, ears, nose, skin. Sight, hearing, smell, touch. Of everything your surroundings hand you at any given moment, you use a sliver and let the rest run past. Sensus Altus opens those channels, amplifies them, and speeds up your mind for the short window it lasts."
She listened, rapt.
"The direction air moves. The tell in an opponent's muscles before they commit. Magic gathering at a wand tip before the spell leaves it. Details in sound and light too fine for you to register on an ordinary day. Open Sensus Altus and all of it comes in enlarged and early. In a fight, that means you know what he's doing a fraction before he does it."
Her breathing quickened. This was precisely the piece she'd been missing.
"Sounds excellent." He turned it again. "But there are limits. It adds nothing to the power of your spells and it doesn't make you cast faster. All it buys you is knowing a little sooner."
"A little sooner is plenty." It came out before she could stop it, her eyes blazing.
"For you, yes." He nodded, serious. "Give it to someone who charges in swinging and it's wasted. Their mind can't keep up. Yours can."
Warmth spread through her chest. Her mouth started to curve and she flattened it fast.
"There's a price," he continued. "It doesn't last long. Every sense wide open, your mind running flat out, and it eats at you. The first few times you'll be dazed. Too much arriving at once, no time to filter, everything piling up inside. It leaves you worse off, not better."
Her brow furrowed. "So what do I do about that?"
"Same principle as Constant Protego. Learning it is only the start. How far you take it is on you." He added, "And it isn't only for fighting. Somewhere dangerous later on, pitch dark, a wall about to come down, something waiting where you can't see it. A moment of Sensus Altus beats being blind."
She nodded hard, filing it away.
"Pronunciation. Sensus Altus. Stress on the front of the first, SEN, and the middle of the second, AL. SEN-sus AL-tus. Say it low. No need to shout."
He raised his wand, tip hovering at the center of his forehead. "Point here first. Inward, toward where the senses come in, and then..."
The tip cut a short arc outward. "Open it. Spread the senses out. Two motions, that's all."
He lowered the wand. "Try."
Lily brought her wand to her forehead, drew a deep breath, and shut her eyes.
"Sensus Altus."
Nothing stirred at the tip.
She frowned and went again. The gesture was right, the stress fell wrong.
Third attempt, the stress landed, but her wrist swung the arc too wide.
"Smaller arc." He watched, correcting without impatience. "And sink into it when you point inward. Don't let it drift."
Fourth attempt. Wand tip to her forehead, sinking, the words low, a short arc outward.
It took.
She went rigid as the world poured in.
Moonlight was no longer a flat wash of silver. Thin cloud let it through bright, thick cloud dimmed it, and the layers of light and shadow lay across the floorboards in perfect definition.
She saw for the first time that moonlight had color. Cold near the window, edging blue-white, warming as it reached inward, going dull gold where it fell across the old desks.
Every beam's path from glass to floor was traceable, and the dust churning inside each one, thousands of motes, every one of them drifting separately in her sight.
Grit in the wall joints. Scratches in the floor. The crooked row of abandoned desks at the far end. Things that ordinarily blurred into one mass rose out of the dark one at a time, edges sharp.
The night wind outside separated into strands, some scraping the window frame, some slipping through the gap under the door. She could tell the friction of air dragging over stone from the shiver of fabric as the curtain stirred.
Knox shifted by the window, tail tip brushing the floor, and that small sound reached her like silk drawn across the shell of her ear.
Wind caught a corner of Regulus's robe, and the rustle of the cloth was right beside her.
The air carried dust and old wood and the dry mineral breath of the sill's stone giving back a night of moonlight. She had never once noticed that this classroom had a smell.
The whole world had been dragged close. Sound, light, moving air, scent, all crowding her senses at the same instant, sharp past the point of belief.
She didn't know where to look or which sound to follow. Everything clamored for her attention at once, nothing above anything else, no thread to pull.
Her vision swam. Her head rang and swelled until it hurt. She swayed and caught the edge of a desk.
"Everyone's like this the first time. Too much coming in."
His voice arrived from close by, layered in her ears, the part carried through air, the part bounced off the walls, and beneath both the low resonance of his chest.
"That's the first hurdle. Your mind hasn't learned to filter. Hold on. Don't shut it down yet."
Lily blinked hard twice, jaw clenched, braced on the desk, breathing slow.
So this was the world. This loud, this full, and she'd been letting all of it run past her.
The spell held, and the flood kept rising, but she found a sliver of herself to work with.
Take hold of his voice first. Push the rest back a little. It wouldn't push far, but it was better than a moment ago.
She breathed deep and gathered her focus, and the chaos began sorting itself into layers.
Her own heartbeat slowing in her ears. The wind outside resolving from a smear of noise into distinct, separable sheets.
Then Regulus's breathing. Even. Long.
"...that's a lot of things." She was panting, her voice thin.
"First time casting it, the spell's weak and you're not practiced." Laughter had crept into his voice. "That little strength of it was enough to flatten you. Once you've drilled it and the spell's stronger, more will come, and finer. If you can still pick out what matters then, you'll have learned it."
He studied her face. "Age helps too. Once you're older, your mind steadier and your will firmer, you'll take in more. You're young yet. Getting it to work at all is already good."
She pushed herself upright by degrees. Barely fifteen seconds and her temples were damp, her chest rising and falling.
She held it, teeth set, a few seconds more, then let the magic go. The spell dissolved.
Moonlight flattened back to silver. The wind receded. The smells vanished. Knox's tail made no sound at all.
The world snapped back from unbearable clarity to its ordinary self, dimmer, slower.
Lily let out a long breath and sagged.
"...that's exhausting."
Still catching her breath, she wiped her forehead. Her hairline was soaked.
Once her breathing evened out she straightened, color high in her cheeks, eyes brilliant.
Regulus put his wand away and crossed to her. "Two things tonight. One slow, years of grinding. One quick, usable immediately, but the ceiling on it is yours to earn."
Her green eyes crinkled, something stubborn under all that seriousness. "Fine. I'll take both. Constant Protego in a year, and Sensus Altus until I can hold it."
He looked at the small determined figure of her and gave a thumbs up. "Well done."
Outside, the moon had fought entirely free of the clouds, brighter than before.
Lily was damp with sweat, cheeks flushed, loose strands clinging to her temples.
Regulus turned and pushed the open window shut, sealing the night wind out. Quiet settled over the classroom.
They rested a while, waiting for the sweat to cool.
Nearly ten o'clock.
Lily took her school robe from the hook in the corner and pulled it on. Regulus took his as well.
Knox sat at her feet, tail wound around her ankle, deep blue eyes looking quietly up at her.
She bent and gathered him up. He curled into her, chin resting on the back of her hand, breathing out one small puff of warm air.
She pressed her face briefly to his head, then lifted it, a smile breaking across it.
They walked to the classroom door.
Lily turned, the cat in her arms, red hair loose in the moonlight, her face still bright from the heat of the work.
Her mouth curved. The moon outside the window sat reflected in her eyes.
"Good night, Professor Regulus." Her eyes crinkled at him.
"Good night, Miss Lily." He raised a hand and waved.
She laughed and set off for Gryffindor Tower, quick footsteps carrying down the corridor.
Knox lifted his head from her arms, eyes going over her shoulder to Regulus for one long look, then withdrawing.
The footsteps faded into the passage.