Or sorry, I think I should elaborate
Rulers have an age and an health. That degrades over time, and sometime you can have accidents. The more decadent your nation is, the higher the chance to get an accident (and the weirder the end message will be, you should read what some of the emperors of Rome did, this is sometime so strange you believe it's an invention...).
Once he dies, a new one appears, with new traits, and generally a good health.
A ruler can't gain a trait, for now (we have thought about it, but it won't make for release at least).