Changeling Enchantment

"I think Elena may be a changeling. Inhabited by a fairy at birth."

- Gaius to Merlin

Changelings are individuals inhabited by a Sidhe. They are often unaware of their condition until the Sidhe emerges and possesses them entirely.

Becoming
According to Gaius the body must be inhabited by a fairy at birth.

Unbecoming
There is a Potion which was made by Gaius, first it was created by the worst recipe writers, Witches of Meredor & it contains the stamen of the Dropwort flower so could help Princess Elena to get rid of the fairy.