Buffy had asked the others to meet her at her place after school to discuss what the pompous new Watcher had said. After explaining she turned to Amy, "Is what he's saying possible? Could I be corrupted just by there being two Slayers?"
"If he's right that the Slayer is mystical, it is possible. Magic can both corrupt and be corrupted. I don't smell Magic on you, but all it would take is a spark of Magic to activate whatever it is that makes you a Slayer. Once activated you'd essentially be self sustaining, until death. And mixing different Magic could also have side effects. I didn't stop to think about that, anyone else I revived that way, Jesse or Cordelia or Xander, there'd be no Magic to interfere with, they're natural verses magical. Another Hexenbiest or a Zauberbiest wouldn't be an issue either because our Magic is the same, just differing levels of skill, ability and power levels. And if there's never been two Slayers active at the same time, there may be a good reason for that."
"How do we find out for sure what effect it's had on her?" Joyce asked anxiously, fearful for Buffy. "And how do we fix any problem there might be?"
"I'll go get my spellbook." Amy decided. "There's diagnostic spells in there, I should be able to find something. I can't even guess as to what, but something. Try not to panic, Buffy, Mrs. Summers, we'll figure it out."
"And if Amy isn't able to do it with her Magic, there are other Magic users with other kinds of Magic." Cordelia pointed out. "Someone will have an answer and we will find it. It's like visiting different doctors to figure out why you're sick."
"And if you do turn evil, we'll figure out how to make you good again." Jesse promised.
"And if we find we can't make you good again, I'll kill you before you can hurt, torture or kill anyone and I'll make it quick." Xander promised.
"I find that all, including the promise to kill me to protect others, to be very comforting." Buffy laughed softly.
"Bottom line, we won't just give up on you, B." Faith assured her. "We'll look out for you."
"As they've looked out for me." Tika smiled at her. Amy left and returned as quickly as she could.
"I found a diagnostic spell." She told them. "I got the ingredients, give me a minute to put everything together and then we'll see what there is to see."
Amy mixed several different herbs and other ingredients in a bowl and then set it in front of Buffy and ignited the mixture. After which she recited the spell:
Possim rei huius cognoscere causas (May the causes of this be known to me!)
"Xander," Amy said, staring at Buffy. "You've always said you can see something dark in Buffy."
"So can I." Faith said. "Rory can too, Xander told me Giles said it's the Slayer Essence, which I guess is what makes the Slayer."
"She feels human through." Xander pointed out. "My senses don't register her as a demon, the way a vampire does."
"Same with me." Faith agreed.
"That's because she is human and she's not possessed by a demon. However, she does have demonic power in her. Deeply imbedded into every cell of her body."
"You're saying the Slayer Essence comes from a demon?!" Buffy exclaimed.
"Yes." Amy answered. "It's how they make the Slayer, with power that came from a demon. It would make you sensitive to other demons."
"Able to sense them, you mean." Buffy asked.
"That's why you use your mind to sense them, Buff." Xander said. "It's probably tied to a psychic ability."
"That would explain how and why Slayers have prescient dreams." Amy agreed. "And why you remember things earlier Slayers knew. Whatever it is that sparks the activation probably takes an imprint from each Slayer. The Potential may have been tied to a pre exiting prescient ability. It also explains why and how you might become corrupt. If something moved on from you in order to activate another Slayer, that something could have helped provide a moral balance in you."
"You are going to have explain that because I'm very confused." Joyce said.
"The same thing that blocked Buffy from sensing the demon in that vampire." Xander said softly, putting the mental puzzle together. "The spark is a piece of the soul of the previous Slayer. The soul remembers every lifetime. I can't see souls, but I can see demons. I saw the demon in that vampire, Buffy was blocked from seeing it by the soul it was cursed with."
"A piece of Buffy's soul has been taken from her?" Joyce asked, horrified. "And used to spark this demon power in this new girl. Why?"
"It would reinforce her own soul and protect from the corrupting influence of the demon power." Amy answered. "And the loss of it erodes Buffy's own soul's ability to protect her from the corrupting influence of that power."
"How do we fix this?!" Joyce demanded.
"One of two ways." Amy said. "One is to tie what's left of her soul to someone else. There are pros and cons to that. Pro, the other person's soul can help stabilize her own and provide some protection. Con, if the other person went evil voluntarily or involuntary, Buffy would be dragged with."
"And the second?" Buffy asked.
"We cleanse you of the demonic power. It's possible that you'd cease to be a Slayer entirely without that power. Unlike Xander and Faith, who are genetically enhanced, that demonic power is what is enhancing you."
A/N Diagnostic spell from Supernatural wiki.
This type of soul bound appeared in an episode of Charmed. Pru was forcibly tied to a demon, which corrupted her and through her linked power with her sisters, corrupted them as well, before being reversed.