Cassilda is the queen of Yhtill. She has two sons (Thale and Uoht) and one daughter (Camilla). She bears the Diadem, which appears to be the symbol of leadership in Yhtill.

Cassilda would like to give the diadem to Camilla, but she refuses because she fears seeing The Yellow Sign which is the fate of rulers of Yhtill. The two brothers both want the diadem, and it seems that who gets it depends on which of them Camilla chooses as her husband (Yhtill evidently has some odd views on incest).

Cassilda appears to have the power to change things in Yhtill which is stagnant, but chooses not to.

In Carcosa, or The Queen and the Stranger, Cassilda is played by Hannah Keith.