Which Buffy episode reveals that Dawn was created from mystical energy called the Key?

The Buffy episode that reveals Dawn was created from mystical energy called the Key is “No Place Like Home.”

“No Place Like Home” is the fifth episode of Buffy the Vampire Slayer’s fifth season. Buffy begins investigating the strange circumstances surrounding her suddenly appearing younger sister, Dawn. During the episode, Buffy discovers that Dawn is not biologically the Summers family member everyone remembers her as being: she is the Key, a mystical object sought by the hell-god Glory.

The Key’s protectors transformed the mystical energy into human form and implanted memories of Dawn into Buffy, Joyce, and the people around them. This explains why Dawn seems to have always been part of the family even though viewers first meet her at the end of the season-five premiere, “Buffy vs. Dracula.”

The revelation creates one of the season’s central conflicts. Dawn is emotionally a teenage girl with genuine relationships, but Glory sees her as a means of opening a portal between dimensions. The later episode “Blood Ties” focuses more directly on Dawn learning the truth herself, which is why the two episodes are sometimes confused. “No Place Like Home” is the initial discovery by Buffy.