This is the speech from Buffy which explains what The Scythe is and how it is relevant to the end of the world and The First Evil

WOMAN
(admires the scythe) A weapon. A scythe. Forged in secrecy for one like you who— I'm sorry. What's your name?

BUFFY
Buffy.

WOMAN
No, really. (Buffy shrugs) We forged it in secrecy and kept it hidden from the Shadow Men, who—

BUFFY
Yeah. Met those guys. Didn't really care too much for 'em.

WOMAN
Ahh, yes. Then you know. And they became the watchers. And the watchers watched the slayers. But we were watching them.

BUFFY
Oh! So you're like... what are you?

WOMAN
Guardians. Women who want to help and protect you. We forged this centuries ago, halfway around the world.

BUFFY
Hence, the Luxor Casino theme.

WOMAN
Forged there, it was put to use right here...to kill the last pure demon that walked upon the Earth. The rest were already driven under. And then there were men here, and then there were monks. And then there was a town, and now there was you. And the scythe remained hidden.