Heart of Darkness is a Legendary Source for the Wizard in Diablo III, obtained via purchase of Reaper of Souls Collector's Edition (or Digital Deluxe edition) as part of the Arma Mortis pack. It can only be used for Transmogrification.
Heart of Darkness
"Within the breast of every human on Sanctuary lies a mote of pure darkness, proof of the breed’s demonic parentage. I suspect there may be a way to draw out that evil and contain it. Of course, the process is fatal, but I propose death is infinitely preferable to the life of doubt, rage, and confusion this darkness engenders." — Author Unknown, Armis Mortem
- This source looks like a miniaturized Soul Crucible.