Once that last bell rings, this stupid town will go up in flames, and everybody will go poof.
Silas later revealed to Damon that Katherine's blood became the cure after she had ingested it, which is why he wanted Katherine so he could cure himself and die so he could be reunited with his true love in the afterlife.
She ran from him for weeks, but eventually, Damon helped Silas lure Katherine to the house and fed him her blood, which turned him back into a witch.
She was then exiled to England, where she quickly assimilated into her new culture and met two nobleman brothers, Niklaus and Elijah Mikaelson.
At first, she was attracted to both men, but in time, she eventually learned that she was a Petrova doppelgänger and that Klaus was planning on using her as a sacrifice in order to break the curse that bound his werewolf side.
She sabotaged his plan by running away with the moonstone, which bound the curse, and then tricked Rose into feeding her her blood so she could turn herself into a vampire.
Because Katherine was no longer human, her blood no longer was a viable component of the ritual to undo the curse.
Klaus was so infuriated with Katherine's sabotage, that she was forced to live on the run for over 500 years as he hunted her down.
Her activities during her first centuries as a vampire are unknown, but at some point between the 15th and 19th centuries, she took on the alias "Katherine Pierce" (an anglicized version of her birth name) and eventually met and befriended Pearl, Anna, and Emily Bennett.
With this list of vampire slayers, you'll always know who to call if you find yourself surrounded by bloodsuckers.
I'm not sure when it happened, but at some point in the last few years all of the vampires in fiction - whether it be movies, TV, or comics - became giant vaginas.
When the Founder's Council found out the identities of the vampires in town, she faked her death and escaped being sealed in the tomb with the aid of George Lockwood, who helped her in exchange for the moonstone.