Her character Vicki was transformed into a vampire and subsequently killed off in the CW drama's first season, but made a surprise return in the second season finale.
Ewell told TV Guide: "[Vicki's dealing with] unfinished business with my brother, my boyfriend, my ex boyfriend, with so many people. I'm coming back with a vengeance. It's going to get crazy."
The actress also described her shock reappearance in 'As I Lay Dying' as "the most top secret job I've ever done in my life".
Ewell admitted: "I couldn't even tell my brother and sister because they're both in college and I was really afraid that [it would end up on] Facebook and Twitter. So I literally told my mum and dad, and that was it."
Vampire Diaries star Nina Dobrev recently indicated that the show's third season is "like a blank slate".
"I think it's going to feel like a completely new show now that Stefan (Paul Wesley) has gone to the dark side," she suggested.
The series will return to Thursdays at 8/7c in the fall on The CW.
> Vampire Diaries star 'wants single Elena'