I think it should be important to consider when the Loptous tome is passed down too. I assume only one person can be possessed at a time so whenever the crown prince comes of age they are granted the tome and the current emperor loses their power. I guess it would be during this time that their now-possessed son would kill them or seize power themselves. This would make sense since Julius got the Loptous tome some time before gen 2 begins, so it would have only been a matter of time before he declared himself emperor as asserted direct control, even though he was clearly calling the shots instead of Arvis.