Buffy the Vampire Slayer Season 3 Episode 22

After their ultimate ‘Fight-to-the-Death’ Faith escapes from Buffy, albeit in a coma, so Buffy offers her own blood to Angel to rid his body of the poison. When she recovers the gang finally devise a plan to defeat the Mayor, rallying the students and asking for their help in fighting the resulting serpent demon before Buffy leads him into the library which has been rigged with explosives.