Disclaimer - Buffy and co belong to Joss, Harry and co belong to JKRowling. Unfortunately, I am nothing but a delusional teenager, using their magical worlds for the fun of it.
"So, I guess it's story time, then," Buffy began. It was the next morning, and she and her sister were sat side by side in the kitchen of Number Twelve. They were facing the entire Order of the Pheonix, as well as the younger occupants of the house (Dawn had reasoned that as Harry deserved to know, the rest would either get it out of him, or bug her about it, so they may as well hear first hand).
"Dawnie, can you do the Giles bit? My version ends in blah, blah, blah."
"Do I have to?" Dawn moaned, but at a glare from the blonde Slayer continued, "Alright, alright. Jeez, no need to glare."
She cleared her throat and intoned, "In every generation there is a Chosen One. She alone will stand against the Vampires, the Demons and the Forces of Darkness. She is the Slayer."
"I am the Slayer," Buffy finished.
"Technically, a Slayer these days." Dawn interrupted.
"Anyway," Buffy cut across, "the point of all this is to explain exactly where we've been for the last fourteen years, and why we left. Me being the Slayer is that reason. I was Called during my third year at Hogwarts, the first and only student ever to be Called. I used to train in the Room of Requirement, and patrol the Forest and Hogsmeade at night. After three months, James found out. Three months later, the entire group knew. They started to train with me, and patrol occasionally. It continued that way throughout my years at Hogwarts, with only one part of my life as the Slayer really worrying me – that one line in the book I'd been given. 'Most Slayers die before they reach eighteen'. It haunted me, knowing that every day took me closer and closer to the time limit hanging over my head. Then the War started. I was desperate to fight, right from the start. It was my duty, my destiny, to fight. As soon as I left school, I joined the newly formed Order of the Pheonix, along with my friends. That year my father died, just after my mother became pregnant with Dawn, her 'miracle child'. When Dawn was born, Mum died. Having a baby was too much for her. James and I named her, Dawn Marie Potter. Marie for Lily's, her godmother's, middle name. Dawn for a link to Sirius and Remus, through astronomy and mythology. The six of us looked after her. James and I made a promise the day she was born that we would protect our baby sister no matter what, a promise that James and Lily later repeated for Harry. This was the promise that would force me to take Dawnie and run three years later."
"After Harry was born, we were all so happy. Lily and James walked around for weeks with permanent smiles etched into their faces, and Sirius was nearly crying ever time he looked at his godson, not that Remus and I were any better. It wasn't until a year later that everything went to hell. But not in the literal sense. James and Lily knew Voldemort was after them, and James started trying to get me to leave. I refused, every time. After going under the Fidelius Charm, James contacted the Watchers' Council, making a deal with them. He'd get me to go to them, if they'd let Dawn stay with me. Eventually, I gave in. If I was just my life on the line, I would never have left. But I couldn't let Dawnie die. I just couldn't. I tried to convince James to bring Lily and Harry with us. God, I wish he'd listened."
"We were kept at the Council for six years, almost prisoners. Then one day they said it was time for me to do my duty. They put a glamour on me to make me appear fifteen, and as Dawn was nine, they said it looked 'more natural'. We were placed with a foster family, Joyce and Hank Summers, and moved to LA. That year was my first on active duty, as my first watcher, Merrick, called it. Merrick was killed that year, and I ended up expelled from my high school after I defeated that year's Big Bad, a Master Vampire known as Lothos. The reason for my expulsion? I kind of, maybe, just possibly, burnt down the gym," Buffy admitted sheepishly.
This left Hermione and Harry, the only ones apart from the two Potter girls to know what a gym was, in shock. Hermione just sat with her mouth hanging open, while Harry quickly dissolved into howls of laughter.
"What's a gym?" Ginny asked, confused.
"It's like a classroom, but bigger. You play sports in it," Dawn clarified.
"You burnt down a classroom?" Remus said, stunned, while Sirius, the rest of the teens and a fair few adults including Dumbledore joined Harry in hysterics.
"It was full of vampires!" Buffy defended herself.
"Go on, tell them the rest," Dawn prompted.
"Okay, I'm getting there! Anyway, then our foster parents got a divorce and we moved to Sunnydale with Joyce. I started my sophomore year at Sunnydale High, and within two days of school I was firm friends with Will and Xand, and we'd averted an apocalypse. Giles, my watcher was the high school librarian, so that became 'Scooby Central', our base of operations…"