You get home late one night.
Someone is in your living room, sitting in the shadows.
You reach for the light switch, only to feel your hand batted away by an invisible force.
“Don’t. Turn. On. The light.”
You recignize the voice. It’s Severus Snape.
You sit down across from him. You notice a long mirror, a dimly glowing blue gem, and a black backpack, none of which are yours.
“Aren't you dead?”
“Obviously not. Do you really think I’d get anywhere near Nagini without keeping the appropriate anti-venon on hand? Does that sound anything like a potions master?”
“Hey, that’s, you know, a good point.”
“Obviously.”
“So, uh, what are you doing here?”
He leans forward. You can see his face. He’s worn and tired.
“I hate it there. I want a change. Your world. I want to give it a try.”
“Ah, okay. But, why come to me?”
“Balance. If someone comes here, someone else must go there.”
“Yeah, about that, I’m a muggle, so, can’t really—”
“Stop. Talking. I have already worked everything out. So there’s no need to risk giving your brain an aneurysm. You pick up the gem. When you step through the mirror, you will find yourself reincarnated into your ideal body, except you will be magical, eleven years old, and on the train to Hogwarts for first year classes.”
"But—”
“Of course, you will retain all your memories. Not that they’re worth retaining, much as I’ve been able to discern, but if you leave your memories here, mine go back there. Balance. Don’t try to comprehend. I haven’t got all year.”
"Okay, but—”
“But you want to take your possessions with you. How very mugglish. And so predictable.”
He gestures and the black purse levitates into your lap.
“It has interior space equivalent to a fully grown bull elephant. And it does all the sorting and filing automatically. I didn’t want you to pop a brain vessel figuring out the proper use, so I made one that’s idiot-proof. Now, would you like to take a few hours to pack, or are you going to turn down the one chance you have for magic?”