Yttermayn
The mistake you are making is assuming that the history of Greyhawk that you are familiar with holds true in my world. Some of it may, however, your characters are not privy to all player knowledge in this, or any other, regard. Please don't assume that your character knows anything about iuz himself, beyond that he is a legendary figure of great evil. Now, as far as your character's history is concerned, I have no problem with him having battled the forces of evil in the land known as Iuz, but i do have a problem with characters knowing details about personages of Greyhawk such as Iuz himself. Thanks, and please edit your recent IC post.
I know realize that you [all] may have made certain assumptions regarding this based on your character's levels. Please remember that Greyhawk was written for 1E AD&D originally, where 10th level was extremely high-power for PCs. This being Pathfinder, 10th level is roughly the equivalent of 4th or 5th level in 1E AD&D, at least as far as relative power is concerned vis-a-vis the legendary heroes/villains of the setting.