|This article is about the the Key in its original form. For the being into which the Key was transformed, see Dawn Summers.|
- “The Key is the link. The link must be severed. Such is the will of God.”
- ―Lithany of the Knights of Byzantium[src]
The Key is an ancient mystical energy with the power to unlock the gates between all existing dimensions, though only at a certain time and place. Who, when or why it was created is unknown, though in the XII Century, the Order of Dagon was founded by Tarnis in order to protect it. Meanwhile, the Knights of Byzantium sought to destroy the Key so its destructive power would never be unleashed.
In 2000, fearing that Glorificus would use the Key, the surviving monks of the Order of Dagon transformed the Key into Dawn Summers with the hope that the Slayer would keep it safe. Dawn and everyone around her were given false memories so that they would believe she had always been present as Buffy's younger sister. The monks used Buffy as a template to create Dawn's physical body, meaning that they are biological sisters. People who are "beyond reality", such as seers and mentally ill people, are able to perceive Dawn as the Key.
In 2001, Glory's minion Doc succeeds in using Dawn as the Key, but the resulting portal is soon closed by Buffy's self-sacrifice. Proceeding this, the Key ceases to be significant; in "Bargaining", Dawn states that she is no longer the Key, and even if she is, she no longer opens anything. Additionally, other characters frequently remark that Dawn was the Key (in past tense).
A side note: When Doc used Dawn as the Key and opened the portal, one of the creatures seen to be entering our world through it was a Dragon. It was this same Dragon that can be seen in the final episode of Angel: Not Fade Away. This was done as an "IN" joke by the studio so that Angel could make the comment, "I always wanted to slay a Dragon."
- "No Place Like Home" (Only in flashbacks)