After reviving Aeson, Medea does something wicked. She pretends that she and Jason are separated and goes to the house of King Pelias, who is very old. His daughters welcome Medea, who pretends to be their friend. She tells them how she restored Aeson’s youth, and the daughters ask her if she can do the same thing for their father. Medea agrees. To prove that she can restore youth, she slashes an old sheep with her sword and throws it in her cauldron. A lamb leaps out.
After Medea revives Aeson seemingly out of the kindness of her heart, she proceeds to use her magic to do something wicked. Once she has gained a reputation as someone who can restore youth, she decides to use this to her advantage, suggesting that she had an ulterior motive in her original revival of Aeson: she wanted to gain people’s trust so she could start using her magic for evil.
Four nights later, Medea drugs king Pelias and his guards. She fills a cauldron with plain water and herbs with no magical powers. She then sends for the four daughters. Calling them cowards, she demands that they cut gashes in their father to show their duty to him. Not wanting to fail their father, the loyal daughters start cutting him with their swords. The king begs for mercy. The daughters lay down their swords. Medea jumps up, slits the king’s throat, and throws his body in the boiling water.
Not only does Medea deceive Pelias by pretending to mix the magic potion of youth, but she manipulates his daughters into being the ones to slit his throat. In making Pelias’s daughters betray their father, it seems that Medea is perversely dealing with her own guilt over betraying her father: she wants to implicate others in a crime of betrayal similar to the one she committed.