sunnydaled:

Buffy: When Willow brought me back, it started all of this, right?  This is it, the end is coming?

Castiel: Yes.

When we discovered what she was going to do, we came in search for her, but we were far too late. She succeeded.

Buffy: Right.

Castiel: It’s begun.

Buffy: Then why don’t you just take care of it now? If you say you’re some angel of the Lord, why am I alive? I don’t understand. All I remember is that I was in heaven.

Castiel: This isn’t about blame Buffy, the person who breaks the first seal must be the person to end it. Willow’s messing with power you can’t possibly comprehend. She must stop.

Dean: What are you saying?

Castiel: Only the slayer can stop everything, Dean.

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.