It's probably her heart...?
I guess? But he never loved her. He was obsessed and infatuated. He wanted her for her beauty and her "superior genes" so they could combine theirs together to make the most beautiful and superior genetically children or some shit.
That's plausible! Cause reading back on it, he did say that together they could make a better offspring. But there's also a part where he did say that she was the only one he had ever loved (which can just be him being obsessive with her). My thoughts just went like -- he was probably mad or frustrated because he couldn't ever get her (love and attention) during both times so he probably got it after her death (･_･;
But it'd be so fucked up for the duke to get her head or womb
nothing's impossible for a man who's obsessed （￣へ￣）
Please don't tell me he took what I think he did. (I was thinking her womb/sexual organs but her head might be it too.)