Athamas fled Boeotia after he had killed one of his sons during a fit of madness inflicted by the goddess Hera; his second wife Ino and their other son Melicertes had escaped from his madness by throwing themselves into the sea and were considered dead. So, Athamas went to Phthiotis where he met Themisto and married her. After he realised that Ino was still alive, he went back to her, causing Themisto's jealousy who devised a plot to have Ino's children killed. She had never seen Ino, and when they met, she thought Ino was a servant at the court. So, she told her to dress Themisto's children in white and Ino's in black. Themisto then paid assassins to kill the children dressed in black. However, Ino had switched the clothing of the children, and eventually it was Themisto's children that were killed. As soon as she found out, Themisto committed suicide.
Who was Themisto?
Themisto was the daughter of Hypseus in Greek mythology, the third wife of the king of Boeotia, Athamas. She and Athamas had four children, Leucon, Erythrius, Schoeneus, and Ptous.