Hell Bound - S5-E4
Trivia: Angel leaves the Reaper locked in a metal cage as the camera pans slowly back so we can only see his eyes - almost exactly the same shot as when Connor left Angel trapped in the metal coffin at the end of "Tomorrow". They even say the same thing - "You get to live. Forever". Like father, like son.
Answer: Nope, it's not Darla - that's Rachel from the episode "In The Dark", the third episode of the show (the one with Spike's magnificent opening monologue).
Tailkinker ★