Severus examined the row of bottles before him against the piece of parchment he held in his hand. It was of the utmost importance that he got this right; the security of the Philosopher's Stone depended on it. His defence was the penultimate security measure for the Stone, so even if an intruder did manage to solve it, they would still have one more challenge to overcome.
Come to think of it, Severus didn't actually know what the final protection was. Dumbledore had only told him that he had come up with it himself. He wondered what on Earth could be stronger than this riddle – it had taken Severus months to finalise it. He knew he shouldn't really go through to the final chamber yet, but Dumbledore hadn't sealed it off yet, so he obviously didn't care that much whether Snape saw it or not. Letting his curiosity get the better of him, he swept through the doorway at the end of the room and into the final chamber.
Severus was faced with a flight of stairs down into the centre of a long stone chamber. The focus of the room was a tall gilded mirror that stood directly in the centre. He descended the steps, and as he approached the mirror, noticed an inscription along the top curve of the frame. Squinting at the intricately carved letters, he could make no sense of it. He dismissed it as some kind of gibberish, known only to Dumbledore. His gaze moved down, into the mirror itself. Severus' reflection scowled back at him; this was just a normal mirror.
No, wait. There, only a moment ago, Severus swore he had seen a flash of red. He stepped closer to the polished glass and looked long and hard. There it was again! So it wasn't just a normal mirror then… Now the question was the identity of the red blur. He focused all his attention on his reflection, watching for any movement. He locked his gaze onto a shadowy figure, approaching his reflection from the rear. He could definitely make out the red in this creature, but what was it? As it got closer and closer, he began to notice more detail. It was a woman, with pale skin and red hair. She seemed familiar somehow…
The realisation hit him hard. As Lily reached Reflection-Snape, the real Severus clutched onto the mirror's frame for support. He hadn't seen that face in more than ten years, and after all this time, she was still the most beautiful person he had ever met. He stretched out his pale hand and touched the hard glass, stroking Lily's cheek tenderly. A single tear slid down his pale face before he even knew he was crying. His sobs came thick and face and he sunk to the cold stone floor, mourning the love that had never been and never could be.