Why??

well because if u saw the pics in 3x18 she's trouchring damon in which i think would result in her death it's just a bad feeling like that i hope i'm wrong because i love rebekah

Klaus is in 3x20 , so the Originals aren't dead they are linked together and in the scene 3x18 Klaus was there with her I don't really reallllllllly think he will leave Rebekah die!!! ;)

Somebody will die in 3x18, and I had the same thought as you, I have feeling it might be Rebekah.
This is said about Sage: What you'll also discover is her surprising connection to the Original Family, which adds an interesting twist to any future involvement in present-day Mystic Falls. (And makes us re-think the photo of her crying from "A Murder of One.")
It leads me to believe that it might be one of the Originals who dies, and since Sage is helping Rebekah with torturing Damon it means they have some kind of relationship, plus like bdavis98 said it could lead to her death, maybe Damon will want revenge or maybe Stefan will kill her while he saves Damon, or who knows.
But yeah, I have a feeling it might be Rebekah, but I really hope I'm wrong.