"In my opinion, this is THE single saddest line in the whole series. Sadder than Arthur's, goodbye, sadder than Freya's or Gwaine's or Morgana's leave. Nothing breaks my heart more than when Arthur thinks Merlin would never, could ever lie to him.
Merlin (BBC) - I love how Arthur always says to Merlin he's the worst, and Merlin understands what he really means, but when it counts, when Arthur isn't sure if Merlin will make it, he says what was the underlying message the whole time.
My favorite story was about a boy who'd live forever but his eyes were heavy with the weight of all he'd seen. / The opening lines from the Doctor Who episode "A Town Called Mercy" for Merlin. This fits a little too well.