I know what it does, but when does the plot tell you what it is and where to use it? Granted you get a vague item description, but is that all?
The best hint lies in the item description:
Manifestation of ashen mist received from the ancient dragon. The magic of the ancient dragon allows one to delve into the memories of the withered."
Throughout the game you run into multiple interact-able giant corpses, one of which happens to be the only thing of significance behind a door that requires the King's Ring to access. Without the Ashen Mist Heart interacting with the corpses hints at coming back later (everything in the game that is interact-able has a purpose, there are no 'flavor' interactions that I know of, only ones that tell you do not have the required key. Either explicitly or implicitly).