I think this is it.
Daddy Dearest by smlcspike
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=26139&warning=NC-17Summary: Sixteen years ago, William the Bloody (also known as Spike of Aurelius) and his mate Drusilla decided to enlarge their family. Spike chose a woman called Joyce as his childe, before changing her he decides to get to know her a little better and they create a child together. A baby girl by the name of Elizabeth. Spike decides to leave Joyce to the living and goes away with Dru, only coming back every so often to check on his daughter as she grows. What happens when Spike and Drusilla come back to America after three years to find that Elizabeth is now a vampire slayer. How will Buffy's watcher take the news that his slayer is the daughter of a most feared master vampire and also how are they all going to keep the council from finding out, especially when they send another watcher and slayer from England to investigate how Buffy is still alive after being killed by The Master. This is a different twist on the Spuffy relationship, it's a story of the love between a father and daughter. Nothing more
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