I can not remember the title or author, but the story began with Spike and Xander somehow ending up in an alternate world/realm.
Spike has to take on the role of protector and hunter, especially when they first appear in the new world, because Xander becomes hurt/ill due to the air or overall environment.
The more time they spend in the world, the more Spike hunts, and finds that primal urges involving his vampirism, and desires towards Xander, are rising increasingly.
Gah! Where is it! I stumbled across this fic over a livejournal memory links, but now, nada. Zip!
*whimpers* Come back to me, my darling.