Dark Soul

This is a My Babysitter's A Vampire Fanfic which starts at the end of season 2.

Ethan's still upset because he thinks that Sarah didn't like their date.
Besides everyone is criticizing him because he got Jesse back from his banishment.
And as if that hadn't been enough a new girl shows up in White Chapel.
A girl possessed by a darkness that scares even Jesse sometimes.
A girl with a power that may could destroy everything...

So it seems like Ethan and his friends have to face their yet most evil villain.
But is the mysterious girl really as evil as she pretends to be?