SpellsCurse
Imperio
Imperio is the Imperius Curse, an Unforgivable that places the victim under the caster’s control. In the fourth-year class, students watch a curse make a spider perform tricks. Barty Crouch Jr keeps his father under it. Harry proves unusually able to throw the curse off when it is used on him in class and again in the graveyard.

First mention
Harry Potter and the Goblet of FireChapter: The Unforgivable Curses
The class watches the curse used on a spider, and Moody then casts it on the students in turn.