Season 7 fic- Potentials are activated but it drains power from Buffy.

I have had a scene stuck in my head for a long while but cannot remember anything else from the story. It must be season 7 because they have decided to activate the potentials. Buffy and the others are walking down steps into the hellmouth when she stumbles a bit. When the potentials are activated it drains Buffy a little. Spike was angry because she didn't tell him that was a risk.

gates of time trilogy authoressnebula

That's it. Thanks