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 young 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.