Please help me! Iread this fic years ago on fanfiction.net or here!??...but,I only remember the plot of the story:

Buffy dresses up as a peasant girl for Halloween, then she sees spike (of season 1 or 2, I think) and remembers him as her mate William, who was killed like 300 years ago...she made a deal with a Hell's king (Abraxas, I think), that in exchange for William to live she would give him the slayer's strength (which means she will die),the deal was made in blood...Then she finds out that the Hell's king reincarnated them both in the future but he never meant for them to remember each other....finally Spike remembers  and he and Giles agree to turn buffy to save her,the problem is no slayer has ever been turned successfully before, so for it to work the daemon that gives the slayers her strength most become dominant over her human counterpart...

Please help, it is a great fic...

I believe that is "Echoes" by Holly, and unfortunately, you won't find it anywhere.  She took the story down to turn into original fiction, the only thing I can tell you is to contact her and ask if she would be willing to give you a copy.

Thank you!!! I will try to see if I can get a copy, I really love that story....