Childe and Sire Saga by Kallysten
It starts with His Childe and goes thru three other long sequels as well.
http://fanfic.kallysten.net/menu.html Summary: Set in Season 6, canon until As You Were. First part in the Childe & Sire Saga. "She understood, from the light in his eyes, that this was it. Her time was up. The Vampire Slayer was about to become a Vampire herself."
It's also at Nocturnal Light, but the final story isn't there, just the first three.
Embracing Darkness by SlayrGrl
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=34781&warning=NC-17Summary: There is one thing that Buffy Summers has feared since she became the slayer. It is her worst nightmare, and now she must face it. After rejecting the Shadowmen's offer to make her more powerful Buffy realizes that she isn't strong enough to win in the fight against the First. Now there is only one option left to give her the strength she needs. To win Buffy must die, but can Spike bring himself to damn her?