Themisto

From Greek θεμιστος (themistos) which means "belonging to the law", or "belonging to the customs". It is therefore related to Themis.

In Greek mythology, Themisto, daughter of Hypseus, was the third and last wife of Athamas. According to some sources, she had four children by him: Leucon, Erythrius, Schoeneus, and Ptous. In other sources there were but two: Sphincius and Orchomenus or else Schoeneus and Leucon. Some say that the father of Leucon was Poseidon

According to Euripides, Themisto wanted to kill Athamas' children by his second wife Ino, who he'd assumed to be dead but was in fact on Mount Parnassus with the Maenads. Athamas had her brought home but kept her return a secret; Themisto did find out she was back, and resolved to kill Ino's children as an act of revenge. However, she mistook Ino for a servant, and ordered her to dress Ino's children in black and Themisto's in white. Ino switched the colours over and Themisto killed her own children by mistake. Upon discovering that, she killed herself. According to Pseudo-Apollodorus, however, Themisto married Athamas after the death of Ino, and the whole story with the murder of the children did not take place.