Prologue:

Rewritten summary: Read The Prologue First.

Karen died and was reborn; taking the place of Elena Gilbert, and she discovers she loves Damon more than Stefan, but is too afraid to take Damon's Route, so she pushes him away, but never got intimate with Stefan.
Now Damon desires her as his and will fight tooth and nail to steal her from Stefan.
Both brothers desire her, and will fight over her.

{This fanfic will go by the books and the show of The Vampire Diaries.}