Amun told him the truth. In the past, he was the eldest son of a royal family and possessed everything: knowledge, power, and authority. He surpassed everything in his world, and he aspired to become the pillar of his family and elevate its status.
But in the end, he became nothing more than a tool of exploitation. His father did not treat him as a son; he was merely a god to be exploited. The world also exploited him to conquer planets and kill enemies. They lied to him.
His lover was with them too. But as time passed, everyone became afraid of him, as there were no longer things they could use to control him. They grew tired of lying to him, and they wondered: what if he stopped believing them one day? He would become a threat to them all.
Hence, the idea came from Kara and his father (because he was the right-hand man of the Emperor King of the World of Light, Alekos): they wanted to transform Amun into a tool, sealing his soul inside an ancient artifact so they could comfortably use his energy.
When Amun found out, he turned against them all, fighting everyone in the kingdom, and the result was inevitably in his favor. Then he clashed with Kara, his friend, and his other friends, and it was then he learned that everything had been just a lie: his mother, his siblings, and his father.
Amun rushed towards Kim and struck him, forming a large field (or 'barrier') so that no one could enter or hear anything.