What is the "ultimate power" that rules the world of tragedy?
This question is a matter of opinion. In other words, all scholars will offer a different answer. In my opinion, what controls the world of tragedy is the idea of "hubris." When there is hubris, the gods intervene, because this characteristic goes against the foundations of the Greek world.
At the center of the Greek world was the Oracle of Delphi. The Oracle had a few fundamental maxims for all Greeks to obey. Two of them were: "Know Thyself," and "Nothing in Excess." These two points were to show that men had boundaries that they must not be transgressed. Man must know that he is not divine. Also man must know to keep within human boundaries. In this sense, these maxims were warnings to the Greeks.
Based on this point, if you examine tragedy, all tragic heroes have a flaw. This flaw is excess and not knowing their place. In other words, they are going against the two maxims of the Oracle of Delphi. Consider Oedipus, who thinks he can solve any problem. He thinks he is godlike. This hubris led to his downfall.