The Slayer line
A certain amount of fanwankery is still required, of course, but here's what the writers tell us...
The First Evil has returned to exploit a disruption in the slayer line. The disruption is a result of the resurrection spell Willow cast in Bargaining. The slayer line now passes through Buffy. The First Evil wants to end the slayer line and become corporeal, to pass into any human as he does into Caleb. The scythe passes demon energy through itself. The slayer is required to make the magic work. All these things are written into the text... Now try this. Within that framework ask yourself all the unanswered questions of Season 7
I posted most of this elsewhere, and unfortunately ran into someone without linear thinking - rather than argue the points ad infinitum, we agreed to disagree. Part of this disagreement is regarding the number of deaths for Buffy and the spell in "Bargaining"
Since you ask the questions below I'll answer them, but not in the order you ask them.
The first necessary part is establishing history of events:
Buffy's first death - mundane drowning and mundane resurrection.
--- Unprecedented event; a new slayer is called. All is ok with the universe ---
Buffy's second death - mystical death and mystical resurrection.
--- Magical price to pay; creation of a demon. All is ok with the universe ---
Buffy's third death - mundane gunshot and mystical resurrection.
--- All is not ok with the universe and things begin to fall apart ---
This is against the laws in several aspects.
1 - It is specifically forbidden as evidenced with Willow's attempt to revive Tara.
2 - It is specifically forbidden as evidenced by Angel's attempt to save Darla who has already been given life once before.
3 - It was not accomplished by going through the "proper channels" but by dark magic alone.
Death 3 - Lawful supernatural resurrection using the Urn of Osiris
followed by an unlawful supernatural resurrection by Willow using Dark Magic. The slayer line regarding passing of power is now in flux as long as Buffy remains alive - the mojo is mucked-up, man! While mucked-up no new slayers can be called (or so The First believes; who may have more intimate knowledge of the process than anyone or anything else).
How is the First Evil planning to exploit it?
By killing all the potentials, then Faith and lastly Buffy so that the mojo has no place to moto when Buffy is eventually eliminated. The mojo would then be obfuscation free - but be all dressed up and have nowhere to party.
What does the First want?
The end of the slayer line.
Why does the First Evil need Buffy alive, but it isn't fussy about Faith?
See above. Potentials first. Remove the base of the pyramid and the structure falls, including the Watcher's Council. Then Faith as the last of the lawfully called slayer line and then! take out the reason The First had the opportunity in the first place to completely sever slayer involvement in any future plans.
Why does the First Evil need a souled vampire the slayer has affection for?
What an interesting question!
If I had to take a guess, it would be in the endgame for Buffy's demise.
Related conclusion at end of post.
You see here's my frustration. Something makes sense. Why is there this need to reject it?
Well, hopefully I've avoided fanwankery and just postulated on evidence as given on screen.
Evidence:
The Trial:
Jeeves: "This is"”very embarrassing."
Angel: "What is?"
Jeeves: "Not to mention unprecedented. She's"”she's been given new life once before"”by supernatural means, yes?"
Darla: "They brought me back."
Angel: "What are you saying?"
Jeeves: "I can't help you."
Angel: "We had a bargain. She's earned a second chance."
Jeeves: "She's living her second chance. But you played the game magnificently. Perhaps you should have told us that she was brought to life"”before this all started. I truly am sorry, sir. The fact of the matter is there is nothing I can do."
---Giving supernatural life after supernatural life = big no-no.---
Villains:
"Xander: You've got to stop doing this. This dying thing's funny once, maybe twice."
---Buffy's third death.---
Demon: You may not violate the laws of natural passing.
Willow: How? How is this natural?
Demon: It is a human death, by human means.
Willow: But I"”
Demon: You raised one killed by mystical forces. This is not the same. She is taken by natural order. It is done.
---Supernatural resurrection from supernatural death = ok; or else the Urn of Osiris never really had any true lawful function to begin with. Supernatural resurrection from "human" death (a bullet for example) = not ok. See Joyce's resurrection for details on just how wrong this is.---
Showtime:
Giles: If The First has been around for all time, then why hasn't it attempted something like this before? Why now?
Beljoxa's Eye: The opportunity has only recently presented itself.
Giles: Opportunity?
Beljoxa's Eye: The mystical forces surrounding the chosen line have become irrevocably altered, become unstable, vulnerable.
ANYA: Something The First did?
Beljoxa's Eye: The First Evil did not cause the disruption, only seized upon it to extinguish the lives of the chosen forever.
Giles: Then what has caused the disruption? What"”what is responsible for letting this happen?
Beljoxa's Eye: The slayer.
Chosen:
The First: None of those girlies will ever know real power unless you're dead.
The First: I will overrun this earth. And when my army outnumbers the humans on this earth, the scales will tip and I will be made flesh.
---The First's plan, and its cause. Slayers have this nasty habit of "keeping the balance" as the Watcher's know full well, and document; mostly.---
---The writer misdirection---
Anya: Yeah, I just"”I don't understand how Buffy's death mucked up the whole slayer mojo. You know, it's not like she hasn't died before.
Giles: It's not because she died. Beljoxa's Eye was quite clear about that in its enigmatic way. It's because she lives. Again. Buffy's not responsible for that.
Anya: Oh. Oh. Willow and me and Xander and Tara. We're the ones who brought Buffy back. We're"”we're the reason The First is here, the reason those girls were murdered. No, it's our fault. The world would've been better off if Buffy had just stayed dead.
---Possible ramifications for season 8---
The First: Oh, no"¦ ow! Mommy, this mortal wound is all"¦ itchy.
The First: You pulled a nice trick. You came pretty close to smacking
me down. What more do you want?
---Pretty close?---
Why does the First Evil need a souled vampire the slayer has affection for?
If the natural law has now been broken, can Buffy no longer die of a mortal wound? And/or, would she need to be turned into a vampire (best chance by someone close and trusted) and dusted to kill her once and for all?