I just finished a run as Mu'nis to go for the achievement, I formed a custom empire consisting of Jerusalem, Armenia, and Syria. I had just won a war to reach exactly 100 realm size, and Mu'nis died about a week after that. I wasn't given the achievement and was wondering why, as I completed the objective during his lifetime. Do custom empires not work? Does realm size have to be >100? I figure I must have done something wrong, but was looking for some insight as to why this happened.