The Vampire Diaries, Season 4

Elena:
(about the visions from killing a hunter) You went through this?

Klaus:
Yes, I did. For fifty-two years, four months, and nine days. I was tormented, in my dreams, my every waking moment, relentless, never ending torture. It was the only period of my life when I actually felt time.

Elena:
So, you knew that this would happen if Connor died? That's why you got involved. Did Stefan know too?

Klaus:
All he knew was that the hunter had to be kept alive. You should have listened to him when he said he had it covered, love.