This entire episode was spent luring Katherine to the Salvatore mansion so the gang could kill her once and for all with the Traveler knife.
And that's actually what happened!
They abducted Nadia in order to force Katherine to show up, we were treated to an emotional scene between mother and daughter, Nadia died - and then Stefan stabbed Katherine.
She later told Bonnie that she stabbed Elena with an extra strong dose of the Ripper Virus and then she was unable to go to The Other Side.
Because she was swept up and taken to Hell instead! That's how the hour ended.
Katherine is a passenger in Elena, except it's worst… because none of us seemed to notice!Caroline
If I had known I just had to die to drag your attention from Stefan, I'd have tried it a long time ago.Nadia