This kind of ran away from me... Six chapters, approximately 12,000 words. I think that qualifies it for the 'long' section of the challenge. :-)
We Just Declared War
Summary: Buffy has hunted demons before. She's seen her share of killing and death. But when she crosses through a portal to confront her nightmare, she discovers that this time, it's not just another big bad to fight. Not just a battle. This time, it's war.
Timescale: Set between the end of Season 7 and the start of Season 8.
Rating: R (violence, suspense horror and profanity)
Wordcount: approx 12,000.
Highlight to reveal what this is a crossover with, if the subtle clues haven't already told you:** The film 'Aliens'. The story starts about two-thirds of the way into the film. **
Link to Chapter One