The actor, who plays Klaus, said that the CW's May 10 episode will have a "lot of shock value" and leave fans divided.
"The last 20 seconds of the episode will leave people with their hearts in their mouths," he told TV Guide. "You're going to be desperately wishing to fast forward and be on to the first episode of season four, because it's hugely dramatic.
"There's a lot of shock value, and it's going to change the world of The Vampire Diaries forever. You should be scared! I think not everyone's going to like it; it's going to provoke some strong opinions. It's really going to get people talking, and that's what we want."
Morgan promised earlier this month that the Vampire Diaries finale will be "epic" and "dramatic".
Executive producer Julie Plec said more recently that the episode will see Elena (Nina Dobrev) make a big decision.
The Vampire Diaries continues Thursdays at 8/7c on The CW. UK viewers can watch it Tuesdays at 9pm on ITV2.