So there are many different styles and opinions as to what you want a prologue to accomplish. Some authors use it to introduce the characters. Others to give the setting, while others use it to present some history behind the story, and others still use it to buildup and legitimize future plot devices. So what do you try to accomplsih with your prologue? I personally try to make it a brief history, but not an info dump. I'll choose characters who predate the setting of the actual story and tell the history through a brief interaction of these characters.