I think nearly every Snape author has, at some point, written the Mirror of Erised story. Here is my offering-and please, everyone bear in mind-I don't like Lily Evans...
The moment he heard of its presence, he was tempted. Of course he was. Everyone was. The greatest weakness of a human being was to know who they truly were. And a mirror that claimed to show the heart's greatest desire...
When his colleagues brought it up in the staffroom, he tried to ignore them. He did not want to look in it. He did not have a heart's desire. He did not need one.
Albus was particularly careful to quash any rumours of the mirror's location. He treated the staff as if they were his students-warning them that the mirror was dangerous. It could cause even the sanest man to go mad. And he asked them not to look in it.
Funnily, that was what tempted him all the more. He had always liked those things that were forbidden-hence his fascination with the Dark Arts and all things shadowy. But he nodded along with his colleagues and promised that he would not go looking for the mirror.
And he hadn't gone looking for the mirror. He had just happened to...come across it...on one of his nightly patrols. He liked to be thorough in his patrols, and that of course meant checking every single room. Even those that were very likely to be empty. Just in case. He was a spy, he needed to be cautious.
So the fact that he was standing in front of the Mirror of Erised, albeit still covered with a thick cloth, was absolutely accidental. But he was here now, and surely there would be no harm in having a small look. Whatever it showed could not be too bad. He knew himself, he knew what he wanted, he knew his innermost soul. He reached up and pulled the cloth from the mirror. It was about as beautiful and magnificent as he had heard the stories tell; an ornate gold frame and clawed feet. It was many feet taller than him. He took in a deep breath and raised his head, looking at his reflection.
He saw himself. The slightly haggard face with the lank dark hair and the black eyes and the hooked nose and the grim mouth. Black robes that hid most of him away. He swallowed and looked at himself again. It was almost an exact duplicate...except after a moment, the reflection was different.
And when he looked closer, he could see that his reflection's shoulders were not hunched. His head was held high. He seemed relaxed.
And he was smiling. Undeniably smiling. His eyes were sparkling. He was happy.
The real him raised his left hand and pressed it to the cool glass of the mirror. His reflection did the same. The sleeves of their cloaks dropped back slightly. He noticed the absence of the tattoo instantly. He pulled back-and this time, the reflection did not copy him.
Instead, the reflection looked at him and pulled back his sleeve. His left forearm was definitely bare. And when he looked at his, he could only see the faint outline of the Dark Mark.
That was his heart's desire.
He looked back at his reflection, who cocked his head to one side and raised one eyebrow in a perfect imitation. He swallowed and his eyes burnt with something hot. "I'm sorry." He murmured.
The reflection raised his hand and he did the same. He rested his forehead against the glass and closed his eyes.
"I did warn you, Severus, not to go looking for the mirror." Albus Dumbledore's voice floated down toward him. "I was warning you especially."
Severus licked his lips and opened his eyes. He looked at his reflection again, and it was almost as though his counterpart had heard Albus' voice. The smile was back, and he folded his arms into his cloak. Severus found himself staring at him, unable to take his eyes away.
"What do you see, Severus?" Albus was closer now, a few metres away from his left side.
Severus turned to face him, taking a deep breath in to try to cool the tears that were threatening to spill. "What do you think I see, Headmaster?"
Albus looked at him for a long moment. "I think you see yourself. I think I see yourself without a Dark Mark on your arm." He noted Severus' lack of reaction and knew himself to be right.
"I want to repeat the past. I want to undo the mistakes I have made. That is my true desire." Severus looked at his reflection, and frowned when his image shook his head slightly.
Albus was still watching him closely. He, of course, could only see the true Severus. "Are you sure?"
Severus' attention was lost, back on his reflection. "What are you trying to tell me?"
The reflection smiled again. It was strange for Severus to see such an expression on his own face. He hardly ever smiled. He withdrew a roll of parchment from his robes and held it up so Severus could read it.
Albus watched as Severus' eyes narrowed-and then widened-and then finally settled on his reflection once more. He knew what the reflection was showing him. Something clutched his chest as he watched the younger man touch the mirror once again.
Severus closed his eyes and nodded, turning away from the mirror. He looked toward the Headmaster, and murmured, "I understand." And then he was gone, sweeping from the room.
Albus watched him walk away, before stepping in front of the mirror. He knew the image so well of what he would see...and yet, tonight, it was different.
He looked at himself in the mirror. He was stood, face clouded in thought, watching and waiting for something. He recognised pain and suffering in his reflection, and saw the signs of age. He looked tired, as if he had only just emerged from a great battle. Maybe he had.
Could a heart's desire change? Why would he wish to see this?
But then someone else entered the frame.
He watched his reflection smile widely, greet the newcomer with open arms, and then draw back to look at him. His friend nodded once, and his reflection seemed to sigh in relief. And his new friend pulled back his black sleeve and showed him something.
Or rather. Nothing.
And the reflection Albus put one arm around his reflection friend and squeezed his shoulder comfortingly, murmuring something into his ear that looked remarkably like, "I'm proud of you."
The reflection Severus Snape looked up. He looked out of the mirror at the true Albus. Stood there, with his mentor close by, and his true heart's desire achieved after much toil and torment, he looked different.
Then he smiled, and Albus smiled too.