Use Pacify or any of the other "dove" spells to make computer controlled enemies who are targeting you less likely to cast spells at you anymore. It makes them target someone else next to you instead, such as a minion or a questing partner.
Don't use it on a flesh-and-blood opponent or anything that actually has a brain though. It doesn't do anything.
Pacify is supposed to reduce a threat on the target. I have no clue what a threat is, so.... But I can tell you that it was a BAD idea to buy that spell. Never spend points on a spell that you don't know what it does. I hope this helped a little.