SpellsCurse
Crucio
Crucio is the Cruciatus Curse, an Unforgivable that causes intense pain. Barty Crouch Jr demonstrates it on a spider in the fourth-year Defence class. It is used on Neville’s parents, on Harry in the graveyard, and widely by Death Eaters. Harry later finds that casting it well seems to require meaning the pain.

First mention
Harry Potter and the Goblet of FireChapter: The Unforgivable Curses
The disguised Moody puts the curse on a spider in class and names it for the students.