Misplaced

Lydia has a car accident and she ends up in the vampire diaries world as Lydia Gilbert, identical twin sister of Elena Gilbert. But she's not a doppelgänger, she's a siphoner. For whatever reason, while she has all of Lydia Gilbert's memories as if she lived them herself, she has no memory of what will happen. And yet she's aware of who the Salvatore brothers are (and what they are) as well as why Damon is in Mystic Falls. She knows about the tomb and about Katherine Pierce (though she doesn't know she's not inside the tomb) and that Damon needs her help to open it. But that's all, no memories of future events except for one thing: Elena is the doppelgänger and she needs to be protected. But, from what?

Damon/OC(Lydia). Identical Twin Sister Story (but Lydia is not the doppelgänger but she's a siphoner; explanation for why is in the story's notes).