The Key

In Buffy the Vampire Slayer, the Key is a mystical object that a group of monks transformed into human form as Dawn Summers, Buffy's younger sister, and sent to the Slayer for protection from the hell-god Glory. The memories of Buffy and her friends, along with the relevant records, were altered so that they believed Dawn had always existed. Buffy discovers the truth four episodes into the fifth season. The Key's purpose is to open portals to other dimensions, a power Glory wants to use to return home. When Glory uses Dawn's blood to break down the barriers between dimensions, Buffy sacrifices her own life to end the apocalypse, which also neutralises the Key's power and leaves Dawn free to live a normal life.