died in the first season finale, causing the next Slayer to get called even though she came back (artificial respiration from Xander). Then, when the second Slayer died, she was replaced with Faith. Now, the line ends with Faith, not Buffy. If Buffy dies, no new slayer gets called, because she's already been replaced, sorta, by Faith. But if Faith dies, a new slayer will get called -- except that events in the series finale make that a moot point.