I haven't read this story or the sequels but I just thought I would see if this could be it. I remembered the name of the story.
Spike's Will be Done. by TalesofSpike
http://www.bloodshedverse.com/stories.php?go=chapters&no=2287An alternative view as to what might have happened in Entropy if Buffy had given all Willow's spellcasting paraphernalia to Spike for safe-keeping instead of throwing it out. This time round, Spike has his heart set on the idea of casting the "Will Be Done" spell before he even reaches The Magic Box. What will the vampire wish for? And knowing the luck our favourite vampire seems to have, should we be surprised when things don't go quite as he planned?
The first "Book" isn't bitey but later ones are.