The Theatre of Cruelty (French: Théâtre de la Cruauté) is a concept in Antonin Artaud's book The Theatre and its Double. “Without an element of cruelty at the root of every spectacle, the theatre is not possible. In our present state of degeneration it is through the skin that metaphysics must be made to re-enter our minds” (Artaud, The Theatre and its Double). By cruelty, he meant not sadism or causing pain, but rather a violent, austere, physical determination to shatter the false reality which, he said, "lies like a shroud over our perceptions."
On the nature of cruelty (also from wikipedia):
Antonin Artaud spoke of cruelty (french: cruauté) not in the sense of being violent, but the cruelty it takes for actors to completely strip away their masks and show an audience a truth that they do not want to see. He believed that text had been a tyrant over meaning, and advocated, instead, for a theatre made up of a unique language that lay halfway between thought and gesture. Artaud described the spiritual in physical terms, and believed that all expression is physical expression in space.