The Herald (The King's Dragon)

The Herald was a mystical spirit. She was known to try and find a host, some being Hilthe and Amy Pond, the latter being able to resist the Herald's power. She was able to take a corpereal form if the one she used as host wished so enough. She ended up imprisoned by the Regulator and her people after she had lost her mind following the deaths of her masters and tried reclaiming the "dragon" for them. (NSA: The King's Dragon)