Katharine’s rule is haunted by whispers of her being "undead" or "cursed". She is literally possessed by the spirits of dead queens, which grants her power but also forces her into acts of violence against her will.

Two Dark Reigns expands the series' world-building by introducing mainland perspectives and delving into the mythology of the Blue Queen. Two Dark Reigns (Three Dark Crowns, #3) - Goodreads

In this novel, the central conflict moves from the struggle to attain the throne to the struggle to hold it. Queen Katharine, having been crowned, must now navigate a realm that views her with suspicion.