Katherine reveals she has a witch of her own, Lucy Bennett (Bonnie's cousin), who has magically linked Katherine to Elena. This means any physical harm inflicted on Katherine—like being staked—is simultaneously felt by Elena.
Just as the characters—and the audience—think the danger has passed, the episode ends on a massive cliffhanger: The_vampire_diares_2x07
Elena tells Stefan that while Katherine is gone, she still doesn't feel safe and can't be with him right now. Katherine reveals she has a witch of her