The Heart is used to help Corvo uncover runes and bone charms hidden throughout Dunwall, by beating faster when turned toward supernatural objects. In addition, when Corvo squeezes the Heart, it whispers secrets directly into his mind concerning his current area, as it senses things about the world beyond mortal recognition. When the Heart is pointed at a person, it reveals secrets about them to Corvo.
A list of the Heart's secrets can be found here.
The Heart is given to Corvo during his first visit to the Void--the Outsider delivers it personally, calling it "the heart of a living thing, molded by my hands."
Vera Moray hints during the House of Pleasure mission that the Heart was crafted from that of Empress Jessamine Kaldwin after her death. When Corvo encounters Granny Rags in the alley behind the Golden Cat bathhouse, she tells him, "Nothing's been the same since her [the Empress'] death! Poor child! Her spirit lives on – trapped – misused – and for what purpose?"
The Heart itself also hints at such origins through various instances of dialogue:
- During the elimination of Hiram Burrows in Dunwall Tower, the Heart tells Corvo that "we [the Heart and Corvo] have both been here before," when used on the exterior of the building. When pointed at the Regent, it says "I was a fool to trust him."
- When the Heart is used on Havelock, the Heart has noticeable difficulty recalling details of the Empress' death.
- In regard to Anton Sokolov, the Heart claims,"He knew me once. And did much to set me on my path."
- When using the Heart on Daud, it remarks, "Why have you brought me here? Am I to forgive this man for what he did?"
- The Heart is voiced by April Stewart, who also voices Empress Jessamine Kaldwin.
- It is implied by the audiograph Piero's Speculation on the Spirit that the Outsider used Piero to create the Heart in the man's sleep.
- A tweet from developer Harvey Smith claims that the spirit within the Heart is still trapped after the game's end, and will be "free once there's no one marked by the Outsider for whom she feels intense love/protection."