The demon heart is discussed by Finn and Jake in the episode "Wizard." They are wondering what a demon heart looks and tastes like when Reaper interrupts their thoughts and offers them free Wizard Powers. At the end of the same episode Reaper comes once more and offers a demon heart to Finn and Jake for free. It is presumably a demon's heart, although it may be figurative as it is shown with various mouth-like appendages, as well as being covered with spikes.
The Demon Heart appears to be a regular human heart with its normal colorways only it has multiple mouths and spikes on it.
- This item was seen again later in "Issue 2."