After they heard Midnight Duke's assessment of the deterioration of the Undead Boundary and the 'owl phenomena' in the sky, the Evernight Dukes' eyes flickered with a similar light to the former.
Furthermore, it wasn't as if the Midnight Duke was letting his imagination run wild; in fact, the other Evernight Dukes, or anyone with their experiences, would have had similar thoughts.
Moreover, the presence of the two Evernight Kings alone proved that there might be something extraordinary hidden or most likely unraveling behind the Undead Boundary.
The Dread Duchess seemed quite satisfied with this information as well, and once she takes over the first battlefront, she can take her time investigating more. Not to mention, once the Undead Boundary falls, the Evernight Kings will rush inside first.
