|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."
- ―Litany of the Knights of Byzantium[src]
The Key was 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 12th 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 Buffy Summers, the Slayer, would keep it safe. Reality was altered and 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, making them biological sisters. However, people "beyond reality," such as the mentally ill, were able to perceive Dawn as the Key.
In 2001, Glory's worshipper Doc succeeded in using Dawn as the Key by spilling her blood, but the resulting tearing of the veils between dimensions was soon sealed back by Buffy's self-sacrifice.
Following these events, Dawn herself stated that, to the best of her knowledge, she was no longer the Key, and even if she was, she no longer opened anything, while Andrew Wells stated that she used to be the Key; however, Dark Willow's threat to turn her back into an energy ball calls this into question.
- "No Place Like Home" (Only in flashbacks)