Elena and Klaus in “Unfinished Business”

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.