In the time of the Dark Lord, secret organisations such as the Order of the Phoenix were designed to destroy him, at times they were very successful. This group had secret means to destroy him. The Dark Lord wound't let this be the case, so he tortured them for information; only when they had the person begging for death would he finally kill them.

You're probably wondering how the Dark Lord went about this. He did so by means of the cruciatus curse. This curse is of course, unforgivable. If you were to perform this curse you would land yourself in Azkaban prison. This particular  curse takes a very great or powerful witch or wizard to perform. The effect of the curse is a jet of red light. This curse is said to be the most painful experience known to man. 

In order to perform the curse you must think of evil thoughts and moments in your life.