From Harry Potter.

The Imperius Curse is used by a high level spell caster to gain total control over target animal or person. It is possible to fight the spell off, but it requires strength of character. With practice, even a determined child can put up a strong resistance to the spell. It is also difficult to keep people under control for long periods of time, as eventually they will realize they are being controlled, and fight it off. The Imperius Curse is one of the Unforgivable Curses.