I have started some games with the pariden and a custom sovereign. Then I decided to start a new game with another custom sovereign and the gilden race. I have choosen to play against the 4 available race. When I met the Pariden, I was surprized to see my previous custom sovereign has the pariden sovereign.
After looking into the faction_parinden.xml file in my games/Race directory, I found that the SovereignUnitType data was set to a custom sovereign.