ETA: Found! The series is by vinniebatman, and is listed in the author's memories or tags under the 'Paranormal 25 Table'
You can also find a companion story at the series page on TTH archive:
Title:There’s Always a Sneer in Vegas (57 /?)
Fandom: BTVS; Spike/Xander
Warnings: Slash, AU, Post Apocalyptic, violence, non-con, slavery, tentacles,
Summary: It’s been seven years since Angelus killed Buffy the Vampire Slayer. Seven years after Angelus opened a portal to hell on Earth. Humans are food, slaves or on the run. There’s hope only in small band of resistance fighters. However, that’s a hope Xander Harris no longer has. It’s been six months since he was taken in a raid. At first Xander thought he was lucky the demons didn’t know who he was; but then that was before he became property to an Azora demon.
“As always: Feedback makes my heart go pitter-patter.
Ok...this is very rough. No time for second read through. I'm running late but wanted to get something posted before I left for the evening!
Nothing the Same, Book 3
Rating: PG13 - NC-17 Individual chapters will carry specific warnings.
Feedback & concrit: yes, please
Disclaimer: don't own them, never will, just playing with them
Spoilers: Anything from Season 1 on.
Summary: sequel to Nothing the Same & Nothing the Same, Book 2
Previous parts here