I bet the duke was in love at the emperor rather than the empress back then. And the reason he hide the empress' body from the emperor was he was mad, that their relationship didn't progress due to her
And by having her remains, the duke now has the emperor's undivided attention (▰˘◡˘▰)
I bet the duke was in love at the emperor rather than the empress back then. And the reason he hide the empress' body from the emperor was he was mad, that their relationship didn't progress due to her