I Abandoned My Engagement Because My Sister is a Tragic Heroine, but Somehow I Became Entangled with a Righteous Prince (Manga) Vol. 4

While secretly investigating Jill and Duke Berklein, Leia discovers that the duke is using Jill for her skills with poison magic. Not only that, but when Johann rushes back to warn the prince, he's captured by the enemy. With their allies dwindling, Prince Eric devises a plan to lure the duke to a glamorous party and expose his crimes for all to see! What will happen when the duke and the prince, who both staunchly hold on to their beliefs, finally confront each other?!