I think this is it.
Daughter of Prophecy by Ashlee
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=9533&warning=5Summary: What if Buffy was never the Slayer? What if she never lived the life that she was supposed to? In fact, what if Buffy was the child of Angel and Darla? The daughter of two vampires- sent to Sunnydale, when it's discovered that it's safer for her to be away from the dangers her father must face everyday. Growing up away from her family, Elizabeth Angel is known as Buffy Summers. A strong warrior, who starts getting a little too close to the vampire that her father trusted enough to protect her. Complete
There is a sequel but since it wasn't ever completed I never read it.
Daughter of Darkness by Ashlee
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=13859&warning=5Summary: Sequel to Daughter of Prophecy - A year after the events of the first story, Buffy and Spike move to Los Angeles to begin a new life, intent on stepping up and handling Angel Investigations. But when promises are broken, unavoidable obstacles begin to resurface and boundaries are tested, the other has a decision to make. When you go to the darkest depths of your mind, is the one you love strong enough to pull you back? Or strong enough to let go? WIP
I really love this author. She has some great stories.
