Demon

The death of a Monarch creates Demons - the last remnants of mortal ambition and desire to obtain power that the mind that was once a mortal Prince forcibly shed in order to reach the next plane. Demons are beings themed around a Monarch's domain that have incredibly strong volition but no reason of their own, as a counterpart to Angels, who have reason but no volition of their own. Demons are drawn to anyone attempting to use or summon their corresponding domain's Angels, regardless of the nature of an Angel's binding. A person caught by Demon will be permanently altered, and their will and ambition will be replaced with the Demon's own. For the rest of their life, they will be able to cast the Prince-gift of the Monarch whose Demon has taken them over, but they will also overwhelmingly desire to take the Throne for their own. Having had their own ambition destroyed when the Demon supplanted them, they will never be able to become a Prince. If they previously were one, they lose their power immediately.

Trying to take the Throne when you aren't a Prince, very notably, will kill you.