Perses carefully investigated and ordered him to be cast into prison to protect his throne from any potential claimants because he feared the prophecy that he should be aware of Aeëtes' descendants (i.e. Medus).
[8] When Medea came back to her native land in her chariot with the yoked dragons, she falsely impersonated a priestess of Artemis before the king.
She said she could make atonement for the plague to appease the gods and she heard from the king that a supposed Hippotes was held in custody.
Thinking that the alleged Corinthian prince had come to avenge the injury to his father (i.e. Creon), Medea unknowingly conspired and betrayed her son Medus.
So when Medus was brought out to pay for his deceit by death, Medea saw that things were otherwise than she had thought, she said she wished to talk with him which Perses agreed.