"Let's ask her directly, with a truth serum."

Miranda, the second Grand Duchess of the Grand Duchy of Fagor, is known as the "Villain of All Time," with a notoriety that reaches even neighboring countries. At one time she sleeps with her sister's husband, at another time she plots the assassination of the Grand Duke and his wife, abuses her sisters, and plots invasions of other countries -- she has done every evil deed possible, but in fact she has something she is hiding...? The awkward relationship between the villain and the mad king deepens with the help of a truth serum. When these two evil people team up, they become a threat that can shake even a great nation!

(Source: Kodansha, translated)