The brothers were very close, but as they grew, their minds diverged. Starswirl wanted to use his magic for the benefit of Equestria, while Chronos, over time, developed jealousy towards the King of Equestria. Chronos desired to forcefully seize the throne, plotting to overthrow King Equinox.
Faced with his brother's ambitions, Starswirl felt compelled to act. Their disagreement escalated into a physical and verbal confrontation. In the heat of the moment, Chronos attempted to end Starswirl's life, prompting a reflexive curse from Starswirl through a spell.
This curse transformed Chronos into a monstrous being. Overwhelmed with despair and fear, Starswirl had no choice but to banish his brother to the Dimension of Chaos to protect Equestria.
Many years passed, and Starswirl encountered Discord, the Lord of Chaos. As Starswirl looked upon him, he realized that it was his brother — the same brother he had cursed and sent to the Dimension of Chaos and darkness. However, Discord seemed to have no recollection of Starswirl.