The Heartless Giant is the title character of The StoryTeller episode the same name. The giant has long been imprisoned in the bowels of the castle at the start of the tale. However, he befriends Prince Leo and plays on his sympathies to secure his release.
The giant then proceeds to wreak havoc on the land, and turns those he captures into stone. His sole weakness is his heart, which he removed himself so as not to be bothered by emotions and feelings.
When Prince Leo found the giant's heart and brought it to the giant's bowels where the army was, one of Prince Leo's brothers grabbed it and squeezed on it enough to kill the giant. Upon his death, the giant's body turned into a hill. When Prince Leo became an adult, he rephrased the legend to his children stating that he gave the giant his heart back and that the giant left the land alone.