A/N The possibilities of extending this Sunnydale into a full story on it's own is growing on me, because yes, this Xander would probably be very different than the one in canon. As for when Willow became a Slayer...Buffy nearly died more then once prior to facing the Master and the only reason she faced the Master was the prophecy from the Pergamon Codex...
######
"You're a Vampire Slayer." Xander said flatly, staring at his oldest friend or rather, a version of her. "Does that mean our Willow is a Potential and no one knows it? How can they not know it?!"
"The Watchers Council's method of locating Potentials isn't a hundred percent." A woman's voice came from the stacks. Xander looked up and saw a woman in her mid forties. "My name is Anna, I'm the girl's Watcher, I heard you come in."
"So, which one came first?" Xander asked. "In our reality Buffy activated at 15 and died in May of her sixteenth year. She drowned and I revived her. She was following a prophecy from the Pergamon Codex."
"She nearly bled out here when a vampire attacked her the night of the Harvest, a ritual to free the Master. Did that happen in your reality?" Anna asked. "It was just after she defeated the Vessel."
"The ritual, yes, but not Buffy being attacked." Xander told her. "How did she survive?"
"I got her to the hospital, she flat lined there and the doctors revived her. She was gone long enough to Call Willow, which was a surprise to all of us as well, given none of us knew she was a Potential." Anna explained.
"I wonder if the timing of her death had something to do with which Potential was Called." Xander mused. "Given Kendra Young was Called when she died the end of May." Trying not to focus on the fact that this Willow was a Slayer, as in short life expectancy.
"It might, but considering there's never been two Slayers at the same time before, it's possibly an unknowable answer." Anna replied.
"So, did the two of you deal with the Master together?" Cordelia asked, eyeing this other Willow and noting she seemed to have a confidence their Willow was gaining as part her relationship with Oz, but certainly hadn't had at this same point in her life
"He hasn't tried anything since the Harvest." Buffy shrugged. "That was a few months ago."
"So time and reality jump." Xander realized. "You're still 16?"
"Yep." Buffy nodded. "What's the Pergamon Codex?"
"A book of Slayer lore and prophecy that was lost a long time ago." Anna told her. "How did you find it?"
"We didn't," Xander answered. "Buffy's boyfriend knew where it was."
"My boyfriend? What boyfriend?"
"Angel, vampire cursed with a soul."
"I'm dating a vampire in your reality?!" Buffy yelped. "Did my brains leak out of my ears or something? A vampire! That's disgusting!"
"Oh, not a vampire anymore, not that it really matters." Dawn said cheerfully. "Since he left Sunnydale just after Halloween last year and hasn't been seen since."
"How can he not be a vampire anymore?" Jesse asked. "Isn't it kinda a permanent thing?"
"Xander destroyed all the vampires." Dawn said blithely. "And the Magic of the curse Angel was under modified the spell and made him human."
"He was under a curse?" Anna frowned. "I've never heard of a vampire being cursed."
"Kalderash People cursed him with a soul." Xander grimaced. "Nasty piece of black Magic that used a soul to trap the demon."
Anna looked sick, she knew enough about Magic to find that thought extremely distasteful.
"Wait!" Jesse held up a hand. "Did you say Xander destroyed all the vampires? How?!"
"Magic spell anchored to the Hellmouth, it drained the Hellmouth in the process. Only it wasn't exactly me." Xander shrugged. "I went as Dr. Strange for Halloween and someone cast a spell that turned me into him, he cast the spell."
"Dr. Strange?" Jesse had an odd look on his face. "As in the Sorcerer Supreme? From the comics?"
"Yeah, that one." Xander nodded. "And yes, he used the same spell from the comics and because the Magic system is the same, it worked."
"All the vampires, gone?" Jesse's stunned look was mirrored by the two Slayers and the Watcher. "Could we do that?"
"We could study the spell." Anna suggested. "We might be able to, I imagine anchoring it to the Hellmouth to fuel it would require a great deal of skill and power, but it might be possible. Is this the spell you mentioned earlier, the one you offered to teach Willow if she did Magic?"
"Yes. And you really need to figure out how to anchor it to the Hellmouth, the spell holds the potential of corrupting a person's soul if they're not strong enough to cast it."
"Than it must be studied carefully before anyone attempts to cast it." Anna said decisively. "But I will take a copy of the spell and begin studying it immediately. The potential is too much to pass up the opportunity."
"Sure, got a pen and paper?" Anna quickly found the requested items and Xander wrote out the spell from memory. "Of course, you can find the spell in the comics too." He handed the paper to Anna. "We should get going, Mrs. Summers and our Buffy were waiting to see if Dawn managed to hit her mark. Jesse, man, it's good to know somewhere in the multiverse you're not a pile of ashes that dusted on a stake I was holding."
"Harsh and thanks, I'd seriously haunt your ass if you let something run around using my face and killing people."
"Same goes for me."