Frost Trace and the Peeping Eye

The first week of December passed in an eerie calm. The Christmas atmosphere in the castle grew stronger, with garlands of holly and mistletoe adorning the corridors, and the suits of armor sporting comical Santa hats. But this festive warmth seemed unable to penetrate the three feet around Snape.

He was more silent and more inscrutable than ever. Apart from necessary classes, he spent almost all his time in the Room of Requirement's laboratory. But Lin Yanqing keenly sensed that Snape's research focus had subtly shifted. He was still working on improving the Wolfsbane Potion, but he was dedicating more effort to studying Magical Solvents and detection spells with strong corrosive, stealth, or penetrating properties. On his workbench, there were several detailed sketches depicting the terrain around the Whomping Willow and the castle's underground piping system (inferred from certain ancient texts).

He was preparing to enter the Shrieking Shack. Calm, meticulous, but unwavering.

Lin Yanqing did not try to dissuade him directly again. He knew that would only provoke Snape's stronger rebellious instincts. He chose another approach—integration.

He was no longer just an assistant handling materials and delivering tools, but began to participate more actively in these "preparations." He would point out a potentially overlooked blind spot on a sketch, use his Ravenclaw identity to borrow obscure books from the edge of the Restricted Section about the castle's structural changes (under the guise of researching ancient magical defenses, of course), and even offer suggestions for improving detection spells based on the limited knowledge of magical trap detection provided by the System.

His participation was technical, devoid of any emotional bias, as if this were merely another necessary branch of their dangerous research. This attitude made it easier for Snape to accept. A peculiar cooperative model formed between them: Snape was the chief planner and main executor of the revenge operation, while Lin Yanqing was his tacitly accepted "logistics officer" providing technical support and risk analysis.

"Ding dong! (?) System Analysis: Contracted target's obsession level continues to rise, dangerous behavior intent confirmed. Auxiliary mission issued—[Frost Trace]!"

Mission Description: Provide basic safety assurance for the contracted target's "adventure." Create a set of simple early warning magical markers.

Mission Requirements: Jointly create and deploy at least three concealed markers with magic-sensing capabilities with the contracted target.

Mission Reward: Environmental Concealment Dust (small area, short duration) x1, System Points +30.

Failure Penalty: None. (System Cub's shrug emoji: "After all, the host is already trying their best to cover the bottom line~")

That evening, as Snape frowned again at a sketch depicting the root structure of the Whomping Willow, Lin Yanqing took out several pieces of Dwarven Cast Iron Slices, exchanged from the System, with uniform texture, and some specially made magical etching fluid.

"Perhaps we could place these along a few possible paths," he pushed the materials in front of Snape, his tone as flat as if discussing potion ingredient ratios. "The simplest Warning Rune, attached to metal, buried in the soil or stuck in a stone crevice. Any creature with strong magic power passing by will cause it to heat up slightly and transmit a weak fluctuation to the main rune piece here. At least... it will give you a few more seconds to react."

Snape stared at the iron pieces, a sharp glint flashing in his dark eyes. He certainly knew what this meant—Lin Yanqing was not only providing technical support but also laying a fallback for him. This was a silent declaration: I cannot stop you, but I will do my best not to let you die in there.

He silently picked up an iron piece, his fingertips feeling its cool touch. He neither thanked nor refused. He picked up an etching knife and began to precisely carve on the iron piece according to the simple diagram Lin Yanqing provided. His movements were swift, with a suppressed ferocity, as if pouring all his unwillingness and anger into these cold metals.

Lin Yanqing also picked up a tool and worked on another iron piece. The laboratory was filled only with the faint hissing sound of the carving knife scraping against metal. This collaboration was entirely different from brewing potions in the past; it lacked the passion for creation and was filled with cold preparation for impending destruction.

Just as they were focused on this, an extremely faint, almost imperceptible sound came from outside the laboratory door—a space conjured by the Room of Requirement, theoretically absolutely private. It sounded like something soft (perhaps the hem of a robe?) brushing against the stone door.

Both of them froze simultaneously, looking up sharply, their gazes meeting in mid-air, seeing the vigilance in each other's eyes.

Snape instantly became like a startled viper, silently sliding to the door, wand in hand, listening intently. Outside the door, there was dead silence, only the faint sound of wind from afar in the castle.

Lin Yanqing also held his breath, sensing Snape's highly tense alertness through their contract. He subconsciously tightened his grip on the [Environmental Concealment Dust] he had just received as a mission reward.

A few seconds later, Snape slowly retreated and, almost imperceptibly, shook his head at Lin Yanqing. No one was outside the door.

But that feeling was not wrong.

Someone had lingered outside the door. Or rather, something had attempted to spy on this secret laboratory.

Snape's expression became even more somber. He returned to the workbench, looked at the unfinished warning iron pieces, and a cold curve tugged at the corner of his mouth.

"It seems we're not the only ones interested," he said in a low voice, a hint of offended anger and deeper vigilance in his tone.

Lin Yanqing's heart also sank. Those eyes that had peered at Snape on the night of the full moon seemed... closer to them. The other party not only knew about Snape's actions that night but might have even suspected, or detected, the existence of their secret laboratory.

The previous conflict was confined to them and the Marauders. Now, an unknown force, hidden in the shadows, seemed to be quietly approaching.

Snape's plan had not yet been implemented, but they could already feel an invisible net slowly tightening. Over the path of frost, the ill-intentioned shadows from the darkness were cast.