I'm 90% sure the Heart the Outsider gave Corvo is that of the late Empress, Jessamine Kaldwin. The commentary on some people and places made me think something was off, before; and then I used it on Daud.

"The last thing the... the Empress felt was his blade." Never had I seen it speak with hesitation before this.

Then, the kicker:

"Why have you brought me here? Am I meant to forgive this man for what he's done?"

Very interesting and thought provoking stuff.

Yeah that's what I figured during her conjecture on being not alive but not dead... Add in that she only ever felt secure around Corvo, and it makes sense that the Outsider would give Corvo back the woman who loved him in a monkey's paw sort of way.

mmmm never thought of who's heart I was using to find my runes and charm bones but now I know and the outsider is a sick

The voice of the Heart is what gave it away for me initially.... but then other things confirmed it.

Yeah, the moment I heard the voice I was 95% sure it was the Empress's heart. Then, yes, all the little lines at different points in the game. The Outsider is definitely sick and twisted.

Well....Great.
Now I'm never going to be able to pull that out and not feel horribly weird about it.
Not that I didn't feel weird about it before but now...
It's a hundred times worse.

About that... Has anyone tried going back to the Empress body in the void after receiving the heart? I never thought to try that while in there.

Also lines like "We've both been here before" refering to the general area of Dunwall Tower, or "I trusted him, but now I can see how blind I was" when refering to the Lord Regent, also enforce the idea that the heart is indeed the late Empress's...

It probably is her heart. It's definitely the same voice actress; she's listed as such in the credits.

With all this evidence it seems painfully obvious it is her heart. That and the fact the Outsider would do such a thing just to mess with you.