When casting a spell you must remember to harm none. Harm includes manipulation, domination, and attempts to control, physically injure, emotionally harm, or hurt another person or group in any way.

The Threefold Law rewards those who follow the Wiccan Rede and punish those who violate it.

The law states that: All good that a person does to another, returns three fold in this life; harm is also returned three fold.

“An it harm none, as I do will so mote it be.”

It must have been a lawyer that thought this up because it sounds like a contract clause of some type. It can be added to the last line of any spell thus voiding it if it has the potential to bring harm to anyone. I would think that it must void most spells since we can not possibly foretell all the possible outcomes of casting a spell.