I'm pretty sure it was Spike/Xander. One (or both) of them had been captured. I think that they go to Angel for help afterwards, and Angel vows to destroy it. The thing is that when they get there, someone else beat them too it, and they have to make a hasty retreat before the place is filling up with lava.
Ring any bells or did I dream this?
Found: The Stranger Things series by tolerik/deacertes/Esmeralda
I found the series here, and the last part is a WIP that only goes up to 19. I'm pretty sure I read more of it, and I found posts that said that she posted up to 23 in her journal but those seem to be deleted. So any chance anyone has those last few chapters saved somewhere?