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 (a strange organ covered in various mouth-like appendages and spikes) to Finn and Jake for free. It is presumably a real demon's heart since it looks similar to the one of an actual demon.
The Demon Heart appears to be similar to a human heart with its normal colorways, but it is four times bigger and has multiple mouths and spikes on it.
- This item was seen again later in "Issue 2".
- Marceline's heart is revealed to have a very similar appearance at the end of the miniseries Stakes.