It's The End Of The World As We Know It

Buffy realized what she had to do. She turned and looked at her sister, her expression serene.

"Buffy ... no!" Dawn realized her sister's intentions.

"Dawnie, I have to."

"No!"

"Listen to me. Please, there's not a lot of time, listen."

Buffy grabbed Dawn by the upper arms and talked to her, ignoring the tears that started to fall. Lightning crackled behind them.

She stroked the side of Dawn's face, then kissed her on the cheek. Dawn continued to cry.

Buffy turned and ran down the platform as Dawn stood there crying. Buffy dove off the end of the platform and into the portal.

... and found herself standing in front of a half-destroyed brick building.

"I shouldn't be surprised that there is a library in hell."