It's been a while since I read it, but I remember it's a pretty long fic (over a few chapters). Anyway, Xander goes into some sort of hell dimension and comes back older and abused. I remember through most of the fic, Spike dislikes Xander. After a while Xander grows on Spike and he starts liking him. I remember Spike has to take care of Xander when the rest of the scooby gang is out of town. During this time Spike accidentally scares Xander (by vamping out?) and Xander tries to run out the backdoor. At the end of the fic you find out it was Spike that sent Xander to the hell diemension.
If anyone knows what this fic is, I'd appreciate it so much!