Harry Potter

Alastor 'Mad-Eye' Moody demonstrates to the students the three Unforgivable Curses: Imperio (the Imperius Curse), Crucio (the Cruciatus Curse), and Avada Kedavra (the Killing Curse). These three spells are known as such for being the most dangerous and sinister spells ever known to the wizarding world, and their usage on another human being often warrant a life sentence to Azkaban.

Example of:
Dangerous Forbidden Technique