This is also known as the crown. The Demon King's heart is an item that merges with the body of a chosen demon. The heart serves two purposes: First as a symbol of authority and power. That is, it will alter a demon's appearance slightly to make them look more like what a demon King should look like. It also enhances the natural powers of the chosen one. Second purpose is lesser known and is to serve as a prison for a primal being of pure void. This often leads to a demon king feeling a great hunger.
There can be only one Demon King at any given time and upon death their heart is passed on to be absorbed by the next king. Though the truth of the heart or its very existence was long forgotten. One day it will be revealed to the new king and awaken once more.