"A wise man had once said, 'He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you.'
This was the lesson that the First Princess was taught by the knight who trained her. He had warned her beforehand that if she was going to fight the monsters then one day she may become one. The First Princess was taught different way to avoid giving into the temptation of the eternal darkness.
The First Princess had to go through hard trainings, master self control and let go of a lot of things and emotions to become the Warrior, to become the Queen who is strong enough, capable enough and has the power to fight the monsters.