The bitch is back. Katherine Pierce is getting ready to rear her head again in Mystic Falls. The Salvatore brothers were duped by Kai in The Vampire Diaries' "It's Been A Hell Of A Ride," and they just made Katherine their biggest threat. At the end of the episode, it was revealed that Katherine is not only still holding onto her anger towards Stefan and Damon, she's getting ready for her revenge. As of now, however, it appears that Katherine is stuck in Hell, or maybe that's what she wants the Salvatores to believe. So, will Katherine return to The Vampire Diaries? All signs point to yes, and when she does, she's bringing Hell with her.
How did Katherine manage to manipulate herself into Salvatore Enemy Number One, you ask? Well, first it involves a very short-lived victory courtesy of the Salvatore Brothers. After Stefan was tricked by Kai into believing that if he stabbed Cade with the magic dagger found at the Armory, that it would kill Cade and destroy his Hell altogether. Damon, who sacrificed himself to Cade in order to save Stefan and Elena, was able to stay alive thanks to Stefan's good aim with the dagger, Bonnie trapped Kai in a prison world, and everyone seemed safe and sound. And then....Kai dropped a bomb: killing Cade didn't destroy Hell, it just let someone else take over. And that person is Katherine Pierce.
Now that Katherine's in charge of Hell, you can bet she's going to use whatever newfound powers she has to get revenge on Stefan, Damon, and anybody else she laid eyes on during her time in Mystic Falls. It's unclear if, like Cade, she can only be summoned by the Founder's Bell, or if she'll have more flexibility since she's not a witch or siren. Knowing Katherine, she'll probably re-write the rules completely.