Ruler of Yhtill in its dying days. Likely perished in the madness that followed the appearance of the Pallid Mask.
She is presented as a character in the play The King in Yellow.
In the Act III Prologue, the party experienced a series of visions from Xhamen-Dor's impossibly long lifespan, which included flashes of the fall of Yhtill. In the vision, Cassilda was described as standing defiant against Hastur, and having made a major sacrifice on behalf of her people, one powerful enough to give Hastur pause.
In Chapter 99, the Yellow King described the characterization of Queen Cassilda in The King in Yellow in this manner: she seemed to be the hero of the story in Act I, then by Act III she appeared to be an anti-hero, and then by the end of the play she was revealed to be the “villain” of the piece. However, the Yellow King’s memory was severely compromised and he may have been remembering the story from a compromised or incorrect viewpoint.
First Reference: Chapter 61
Other Notable References: Act III Prologue, Chapter 99, Chapter 150 (Part 3)