A Draconian Katra was a mystical mechanical device capable of switching the bodies of two persons. Richard Wilkins left it to his protegé, the SlayerFaith Lehane, in case he was killed. He knew that if he was to die, Faith's life would be a short one. Faith used it to swap bodies with Buffy Summers so she could have a new life and shortly after doing so, destroyed it.
In order to reverse the body swap, Willow Rosenberg conjured a magical Katra to return each Slayer to her original body.