Among the Greeks, in the heroic times, a sort of monarchy came into existence that did not survive.  Those who had invented arts, waged war for the people, assembled men who were dispersed, or given them lands, obtained the kingdom for themselves, and passed it on to their children. They were kings, priests, and judges. This is one of the five kinds of monarchy mentioned by Aristotle,  and it is the only one that can revive the idea of the monarchical constitution. But the structure of that constitution is opposed to that of our monarchies of today.
The three powers were distributed in such a way that the people had the legislative authority  and the king the executive authority along with the judicial authority, whereas in the monarchies familiar to us, the prince has the executive and legislative authority, or at least part of the legislative, but does not judge.
In the government of the kings of heroic times, the three powers were poorly distributed. Those monarchies could not last. For once, the people had the legislation : they could at the slightest whim eliminate royalty, as they did everywhere.
Among free people who had the legislative power, people enclosed in a city, where all that is most repugnant becomes even more so, the masterpiece of legislation is to discern the best placement of judicial authority. But it could not be more poorly placed than in the hands of the person who already held executive authority. From that moment, the monarch became terrifying. But at the same time, as legislation was not his, he could not defend himself against legislation ; he had too much power and not enough.
They had not yet discovered that the prince’s true function was to establish judges, and not to judge himself. The contrary policy made the government of one man alone unbearable. All those kings were driven out. The Greeks did not imagine the true distribution of the three powers in government by one man alone ; they only imagined it in a government of several, and they called that kind of constitution a pòlis.