I'm not 100% sure but I think it might have to do with evolving fairy powers to the highest level. The more your fairy Sim uses their powers, the more powers they have. It kind of works like a skill but isn't recorded as one. You get a message every time your Sim gains more powers. This is the similar with witch, werewolf, and vampire sims. Hope that helps! You can check out the Sims 3-The Sims wiki/Supernatural for more information.
