Katherine-episode still from The Return

Season One
Elena Gilbert bares as a striking resemblance to Katherine and that was the initial reason Damon and Stefan Salvatore were so drawn to her.
Damon revealed that she died when soldiers set moto on civilians in a church in 1864. Which was briefly mentioned during Elena and Stefan's History class. It was revealed that Katherine was a vampire during her stay with the Salvatores and was the one who changed Stefan and Damon.
It was thought that Katherine was alive and has been trapped with 26 other Wanyonya damu in a mystical prison under a church for 145 years until Damon freed those trapped...