Emily is an ancestor to Bonnie. She was a very powerful witch. From what we know, she attempted to save Katherine Pierce who was a very evil and powerful vampire as part of a deal with Damon. Damon was to protect the Bennett family line in exchange for spell to be cast to save Katherine. However, the spell required difficult circumstances. A comet to shield them within a church from a fire, and another comet to free them from slumber. There was no way to save one without the rest. many years later Emily uses Bonnie's body to go back on her end of the deal and keep all the evil vampires buried.