A/N The wish is not going to be undone, from canon we know that breaking a vengeance demon's power center only undoes the last wish and it's been months. D'Hoffryn could undo a wish, but it would require the sacrifice of a vengeance demon and no one even knows which demon granted the wish, there's apparently hundreds of them, each with a specialty and Xander was feeling pretty vengeful when he made the wish.
What is going to happen is...testing of Xander and Cordelia as Diana tries to figure out what they know and how they know it. Whatever the reason why Joss originally had Xander retain the military knowledge was, is something we don't have an answer on, hence speculating that it was intended to be because of some kind of psychic ability.
As for Cordelia, well if you've read any of my other stories you know my theory is a natural ability is why Jasmine manipulated and used her in the first place. And no, Jasmine and her plan are out of the picture. Although I do have a few plot bunnies on my story list that include her, but those are for future stories.
This isn't a rehash of my Whispers in the Dark fic, neither Xander nor Cordelia are going to have months of future knowledge to guide them, mostly feelings, some subconsciously retained knowledge and abilities, that kind of thing and some surprises, that's it.
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"Marcie Ross is a very emotionally damaged young woman." Diana told the now four teenagers sitting at her kitchen table. "Years of feeling isolated and alone, coupled with bullying have caused considerable damage. Add to that she has a natural ability called telekinesis, which is what led her to discover the books Mr. Giles unwisely kept in the Library and her attempt to cast a spell on Cordelia."
"Why did it misfire?" Xander asked curiously.
"Things like your emotions and mental state can affect Magic." Diana answered. "It answers to the unspoken, as well as the spoken. Marcie felt invisible, that affected the spell and caused and kept her invisible. Given her emotional state, power is the last thing she needs. I've stripped her power and bound her ability to cast Magic. Her parents knew she was missing and the police thought she'd run away. She essentially had, her parents are working to get her the help she needs."
"I helped cause this." Cordelia looked miserable. "I never noticed her or how much I was hurting her. I never noticed any of the people I hurt. I didn't want to. Would I have ever noticed? Would I have ever tried to be a better person?"
"Yes." Xander answered firmly. "I've got one of those feelings happening now, I know you would have, did. If we're right and time has been changed, at some point you changed too. I believe that."
"That brings me to the next topic." Diana smiled ruefully. "It's unlikely we'll ever know just how much time has been affected and it's doubtful we'd ever be able to correct it, even if we wanted to. For all we know, things are better now then they were when this change happened. The world could have ended and Magic users may have attempted to change a pivotal moment to keep it from happening in the first place. But somehow, both Xander and Cordelia have managed to keep something, even if it's just vague feelings, from that other timeline. We need to understand how and just how much. For all we know, you two may be involved in the pivotal moment."
"How do we do that?" Jesse asked. "And I've got another question, why don't the rest of us also have any feelings? Or maybe we have some buried knowledge too? Is that possible?"
"It is, it's possible we all do on some level. But Xander and Cordelia either have more or it's more easily accessible for them. Either way, we need to start with testing them."
"I'm going to be testing you too, Xander." Mason told him. "You seem to know a few things about techniques and equipment that make me wonder if maybe you were a hunter in this other life. If so, you seem to have kept some, at least at an instinctual level, some knowledge. It would make training you easier, just a matter of turning that instinct into conscious and muscle memory."
"Okay, I'm game." Xander nodded. "Where do we start?"
"Magically induced hypnosis." Diana answered. "And I'm going to ask questions based on the feelings we're already aware that you've had. Try to understand where those feelings came from. We might, and I emphasize might, find a few memories. The soul is the repository of every lifetime and the subconscious could enable glimpses of it. I'd like to start with you, Xander, if you're willing."
"Yeah, I want answers too." Xander agreed. "And I trust you, Diana. That's not easy for me."
Diana nodded as her eyes misted slightly, she understood how difficult trust was for children like Xander and Faith and being the kind of adult they could trust meant a lot to her. She cast her spell and began her questions.
"You told Cordelia you'd been possessed by a hyena spirit, do you know what happened when you were possessed? Is it different from when you and Faith were possessed?"
"I was the Alpha, I gave my pack a pig and decided I needed a mate. I chose her, the strongest female."
"Her who?"
"Buffy, the Vampire Slayer."