"The First Slayer" - genfic, FRT
The origin of the Watchers, according to the Watchers of the 20th century.
"Thusia": part 1 / part 2 / part 3 - Giles/Buffy, FRM
Prophecy chases Buffy in the Los Angeles summer. Giles thinks he knows how they might elude it.
The second one is set between seasons 1 & 2, so it carries a warning for Buffy's age. I don't warn for the violence, though I think society probably ought to care more about that.